Siman 493, Se'if 1

Shulchan Aruch, Siman 493: the laws that apply during the days of the Omer, containing four se'ifim.
"The custom is not to marry a woman between Pesach and Shavuos until Lag Ba'omer, because during that period the students of Rabbi Akiva died. But to become engaged and to make kiddushin is perfectly permitted, and even regarding marriage, one who went ahead and married is not punished. From Lag Ba'omer onward everything is permitted."
(Shulchan Aruch, se'if 1)

(The accepted wording is "Lag Ba'omer," and some were particular about this because the number 33 has the same gematria as the name "Moshe." However, in the text of the counting itself we say "la'omer," and one also finds the expression "Lag La'omer.")

The missing source and reason in the words of the poskim

"The custom is not to take a haircut until Lag La'omer, for they say that then they stopped dying."
(Shulchan Aruch, se'if 2)

The way of a book of halacha: The Shulchan Aruch gives neither the source for the law nor the reason behind it, and the Mishnah Berurah here likewise does not go into the sources. A book of halacha presents the practical ruling, and clarifying where the custom grew from requires going back to the early sources.
The abbreviated language in se'if 2: In se'if 2 the reason is only hinted at briefly, "for then they stopped dying," on the assumption that the student already knows who is being spoken about, and without spelling out the episode of Rabbi Akiva's students.
The term "the custom is": There is no prohibition here stated from the letter of the law; this is a minhag that Klal Yisrael took upon themselves.

The source in the Gemara

A single source: There is only one Gemara on this subject, in Maseches Yevamos, and even there it comes up incidentally. The Gemara says that from Pesach until Shavuos the students of Rabbi Akiva died because they did not treat one another with respect.
The number of students: The wording of the Gemara is twelve thousand pairs of students, that is, twenty-four thousand.
What is not in the Gemara: The Gemara says no more than that. There is no halacha in it, no ruling and no custom deriving from the episode. In the Yerushalmi the account appears with a variation in wording.
The moral question: What exactly was the failure of respect between them that warranted so severe a punishment? That is a question in its own right, to be discussed separately from the halachic clarification.

(Another question everyone asks: why was this particular tragedy singled out to be commemorated for all generations, when Klal Yisrael has hardly lacked tragedies in every era? Entire books have been written on this, such as Sefer HaDema'os by Shimon Bernfeld, and also the collections of documents assembled by Habermann on the decrees in Ashkenaz, France and Spain, which contain terrible accounts written by the survivors themselves.)

The custom of the Geonim: the prohibition of marriage

The tradition found in the Geonim: Among the Geonim there appears a tradition that because Rabbi Akiva's students died, a certain form of mourning is observed, and it consists of this alone: that one does not marry a woman during these days. There is nothing further in their words.
The reason: A wedding is considered the greatest joy in Judaism. When the Gemara says simply "a feast" it means a wedding, and there is no happier moment. For that reason we do not want to celebrate such a great joy during this period.
On the other hand: Judaism views marriage as a matter of unparalleled importance, a person should not delay his marriage, and even a delay for no reason is considered a problem.

Background: when and where the Geonim lived

Proximity to the events: The Geonim did not live all that far from the event, roughly five hundred to seven hundred years afterward.
Torah moving down to Bavel: The Mishnah was compiled in Eretz Yisrael in the year 4000 (the year 200 by the common reckoning), and at that same time Rav went down from Eretz Yisrael to Bavel and opened the yeshivos there. For that reason he is sometimes called the teacher of the exile, being the first to establish yeshivos there.
The yeshivos of Bavel: Sura, Nehardea, Pumbedisa, and other yeshivos in additional cities such as Naresh. This period lasted from about the year 200 until 500, during which the Jews sat in Bavel in tranquility.
From the year 500 onward: There had been decrees earlier as well, but from the sealing of the Talmud Bavli onward the situation in Bavel deteriorated. Kings arose who were not on good terms with Judaism, and Jews migrated to Italy and Spain and left Bavel.
The continuation of the Geonic era: Even so, the Geonim remained in Bavel, and the yeshivos existed there until roughly the year 1000: some five hundred years of Geonim, with troubles and with yeshivos at one and the same time.

The origin of Lag Ba'omer: Rabbi Zerachiah HaLevi

The ancient book: Rabbi Zerachiah HaLevi, the author of the Ba'al HaMa'or, who lived in southern France around the year 1200, writes that an ancient book came into his hands containing the version that the deaths of Rabbi Akiva's students did not continue until Shavuos itself, but rather "until pros Atzeres."
Two interpretations of "pros": On the one hand, "pros" comes from the root meaning to divide, that is, half. On the other hand, we find the expression "pros hachag," like the expressions "erev Shavuos" or "erev Shabbos," which are not precisely defined and depend on how people speak: "erev Shabbos" can even mean Thursday night.
The proof and the calculation: Since this term is not precisely defined, he brought a proof from a Gemara that "pros" means fifteen days before the date. Accordingly one calculates fifty days minus fifteen, which comes out around the 33rd or 35th of the Omer, and from here Lag Ba'omer was born.
A lone tradition: This entire foundation, that the deaths ceased on Lag Ba'omer, rests on this single tradition alone.

The addition of the prohibition of haircuts

A later stage: About a hundred years later, in those same regions, in France and Spain, a custom appears that one also does not take a haircut, on the reasoning that if one does not marry, one also does not take a haircut.
Not from the Geonim: The Geonim wrote nothing of this; it is a later addition to their custom.

The other side: Ashkenaz and the Crusades

The Ashkenazic dispersion: After the Geonic period the Jews dispersed to Italy, Germany, Spain and the rest of Europe. "Ashkenaz" generally means Germany and the surrounding lands, central and eastern Europe.
The decrees of 4856 (1096): In the very period when the Sages of Spain and France are discussing these matters, the Crusades begin in Ashkenaz, the first of them being the decrees of Tatnu (4856), roughly at the end of Rashi's lifetime, who passed away in 4865.
The course of the massacres: The mob that set out for Jerusalem followed a certain route through Europe, and every Jewish community along the way fell victim, above all the SHU"M communities: Speyer, Worms and Mainz, as well as other communities. Horrifying, dreadful events.

Traces of the Crusades in our davening

Av HaRachamim: This prayer was instituted in the wake of the Crusades.
Yizkor: The custom of Yizkor and the remembrance of souls also developed out of those events. Seemingly it does not fit Shabbos, which is not a day of sorrow, nor Yom Tov, which is the time of our rejoicing, and nevertheless the remembrance of souls was fixed on those days, because of the magnitude of the bereavement that the community wanted to commemorate.
Shabbos Mevorchim of Iyar and Sivan: In most communities the custom was to say Av HaRachamim on the Shabbos Mevorchim of Iyar and of Sivan, because the Crusades took place mainly between Rosh Chodesh Iyar and Shavuos.

The expansion of the mourning and the compromise between the customs

Shifting the boundaries of the period: The Geonim spoke of Pesach until Shavuos, whereas among the Ashkenazim the main period is from Rosh Chodesh Iyar until Shavuos, because that is when the massacres occurred.
Broadening the mourning practices: If until now it was a matter of marriage, and perhaps haircuts as well, here were added a prohibition of wearing new clothes, the question of the berachah of Shehecheyanu, and other matters, all of them expressions of mourning. Not everyone agrees to all of these additions.
A change in tone: In the words of the Geonim the word "mourning" does not appear at all, only that we do not want to rejoice at this time because Rabbi Akiva's students died. Among the Ashkenazim a note of actual mourning entered, communal mourning over great numbers of victims, for they lived through their own era and its events.
The compromise in the count of days: The custom relating to Rabbi Akiva's students covers thirty-three days, and from Rosh Chodesh Iyar until Shavuos also comes out to roughly thirty-three days. They therefore struck a middle path between the customs, and whoever observes the one set of days does not observe the other.

The difference between Sefiras Ha'omer and the Three Weeks

The root of the stringencies: The main stringencies during the days of Sefirah did not come from the custom of mourning for Rabbi Akiva's students, but in the wake of the Crusades.
Questions of a new garment, a new home, a new car: These did not originate in Sefiras Ha'omer but in the Three Weeks, and people who did not distinguish between the two periods transferred the laws from there to here, reasoning that if it is forbidden there it is presumably forbidden here too.
The view of the poskim: The poskim say that these are two different worlds. The Three Weeks are genuine communal mourning over the destruction of the Beis HaMikdash and the exile of Klal Yisrael, and there is nothing greater than that. During Sefiras Ha'omer, although the Ashkenazic dispersion suffered terribly, this is not the entire Jewish world. One must therefore be careful and know what is permitted and what is forbidden, and there are disputes among the poskim to this very day.

Mishnah Berurah: the scope of the prohibition in Ashkenaz

"And there is no distinction between a marriage that is a mitzvah, such as one who has no children, and one who does have children, for such is the custom that spread in our lands, not to distinguish. And as for taking back one's divorced wife, there is not so much joy in this."
(Mishnah Berurah, s.k. 1)

Most of the stringencies come from Ashkenaz: Almost all the stringencies we are studying here are Ashkenazic customs, because the Ashkenazic dispersion is later, and once they had adopted the practice they were stricter with it.
A marriage that is a mitzvah: On the face of it, someone who has no children and has an opportunity to marry, and has not yet fulfilled the mitzvah of peru urevu, should be given leniency. It is possible that the Geonim, who stated their ruling in general terms, would have permitted such a case, and it is possible that they meant a second marriage, except that we do not know what conditions they set.
The halacha: a son and a daughter: The Mishnah Berurah's wording is "children," and the halacha is that the mitzvah of peru urevu is fulfilled with a son and a daughter.
Taking back one's divorced wife: Even though there is a mitzvah in this, they determined that it does not involve such great joy, and therefore it is not on the level about which the Geonim spoke.

The difficulty: how can a mitzvah be postponed for so long?

The question: How could they decree that marriage, which is a mitzvah, be delayed for such a long period of time?
Other customs of this type: There are customs not to marry during the second half of the month, when the moon is waning, since that is not a good sign. There the matter is stated as "some have the custom not to," whereas here it is stated in the language of a prohibition.
The basis for resolving it: The mitzvah itself is not the marriage. The joy is in the marriage, but the marriage is an enabler of a mitzvah, from which one expects the fulfillment of peru urevu. One who delays it is not preventing the mitzvah itself in its own place and time, the way one prevents a person from giving tzedakah to a poor man standing in front of him. Even though the matter is not comfortable, there is still something to rely on, since this is not the mitzvah itself but the means to a mitzvah, and for that reason the Geonim allowed themselves to rule this way.

The approaches of the poskim: Ashkenaz and Sefarad

"It appears to me that there is no distinction between a marriage that is a mitzvah, such as one who has no children, and one who does, for such is the custom, not to distinguish."
(Be'er Heitev, s.k. 1, in the name of the Bach)

The Bach and the weight of custom: The Bach passed away in the year 5400, before the decrees of Tach and Tat, but the situation in his time was already bad. His way is to hold with full force to the custom of his fathers and his fathers' fathers, as we see in the matter of chadash, and therefore here too he draws no distinction.

"When there is a reason, such as one who has not fulfilled peru urevu, or one who has no one to attend to him, we are not concerned for the custom."
(Maharam Lonzano, Sefer Shtei Yados)

Maharam Lonzano: Rabbi Menachem Lonzano, a Sefardi who lived in Eretz Yisrael, in Tzefas and in Yerushalayim, slightly before the Bach. He is not familiar with the customs of Ashkenaz, and therefore where there is a reason, such as one who has not fulfilled peru urevu, or one who has no one to attend to him and needs a helpmate, he is not concerned for the custom.
The Pri Chadash: Rabbi Chizkiyah di Silva, who was born in North Africa, was in Italy and came to Eretz Yisrael, agrees with this leniency. He too is unfamiliar with the customs of Ashkenaz and sees no obstacle to being lenient where there is a reason.
The Rikash: Rabbi Yaakov Castro, author of Erech Lechem, who was the rav of Egypt, wrote that taking back one's divorced wife is permitted.
Two approaches: Among the Ashkenazim no distinctions are drawn and it is forbidden in any case, while the Sefardim find grounds for leniency where there is a reason. The Mishnah Berurah does not cite these opinions, whereas the Be'er Heitev names names, and from that one can see who holds which way.

(Incidentally, in Egypt three men bore the title mari d'asra: the Rambam; some three hundred years later the Radbaz, Rabbi David ibn Zimra, who wrote thousands of responsa and even went up to Eretz Yisrael and returned to Egypt; and his student Rabbi Yaakov Castro, of whom it is likewise related that he went up to Eretz Yisrael and returned. The title Nagid is not a Torah title but a title of leadership, although some of the Negidim were talmidei chachamim. Egypt in that period, the sixteenth and seventeenth centuries, was not considered a place of outstanding talmidei chachamim, although talmidei chachamim were there.)

Sha'arei Teshuvah: mutual influence between the communities

"See Pri Ha'adamah, volume 3, page 48, that an incident occurred in Yerushalayim in which they ruled stringently not to marry on the Rosh Chodesh that falls during the days of the Omer."
(Sha'arei Teshuvah, s.k. 1)

The development: The Pri Ha'adamah is a Sefardi, and about a hundred years before his time the Pri Chadash and Maharam Lonzano in Yerushalayim had no problem being lenient. Slowly the influence of the Ashkenazim comes back in, books arrive, people become familiar with the words of the Bach, and as a result they shy away from leniency.
The conclusion: Mutual influence between the communities is a fact, and that is how these matters develop in practice.

The berachah of Shehecheyanu during the days of Sefirah

"'Because during that period' - meaning, and it is not fitting to engage in much rejoicing; but nevertheless, if some matter presents itself for which he must recite Shehecheyanu, he should recite it."
(Mishnah Berurah, s.k. 2)

The view of the Mishnah Berurah: Anything over which it is fitting to recite Shehecheyanu, he recites even during the days of Sefirah.
The stringent opinions: There are poskim, and not merely laymen, who hold that even during Sefiras Ha'omer one does not say Shehecheyanu, and this was transferred over from the law of the Three Weeks. The Mishnah Berurah does not cite these opinions.

In passing · A practical question about buying a car during Sefirah

The case: A man paid for a new car before the days of Sefirah, and the car arrived at the dealership during the days of Sefirah. The question is whether he should take the car or leave it at the dealership, which will cost him money because it takes up space.
One answer that was given: First of all, one has to clarify what the problem is at all, and the questioner said that he is among those who are stringent not to recite Shehecheyanu during these days.
A note: There are those who do not recite Shehecheyanu over a car at all, and there are those who recite it over everything.
The solution that was proposed: If the car will be used by him and by his wife, the berachah is not Shehecheyanu but HaTov VehaMeitiv, and the stringency stated by the strict poskim was said only regarding Shehecheyanu and not regarding HaTov VehaMeitiv. On that basis he was permitted to take the car.