Before us is Mishnah 3 in the fourth chapter of Maseches Rosh Hashanah, which deals with the enactments instituted by Rabban Yochanan ben Zakkai after the destruction of the Beis HaMikdash. The Mishnah opens: "Barishonah hayah lulav nital bamikdash shivah, u'vamedinah yom echad" - originally, the mitzvah of the four species was practiced in the Beis HaMikdash all seven days of the festival, while outside the Beis HaMikdash it was practiced for one day only.
The basis for the distinction in the verses:
"Ulekachtem lachem bayom harishon" - and the verse goes on to list the four species. This is the law that applies outside the Beis HaMikdash: on the first day alone.
"Usemachtem lifnei Hashem Elokeichem shivas yamim" - rejoicing before Hashem takes place in the Beis HaMikdash, and the meaning is to rejoice with the four species all seven days.
It emerges that by Torah law, in the Beis HaMikdash one takes the lulav and esrog all seven days, and outside the Beis HaMikdash on the first day alone. As for Shabbos: in both places it was originally taken even on Shabbos, when the first day fell on Shabbos.
The enactments of Rabban Yochanan ben Zakkai:
The Mishnah continues: "Mishecharav Beis HaMikdash hiskin Rabban Yochanan ben Zakkai" - Rabban Yochanan ben Zakkai, who lived in that generation, instituted two enactments:
"Sheyehei lulav nital bamedinah shivah" - that the four species should be taken everywhere all seven days, even though by Torah law this was commanded only in the Beis HaMikdash. And the reason for this: "zecher lamikdash" - a remembrance of the practice that was followed in the Beis HaMikdash.
"Vesheyehei yom hanef kulo assur" - that the entire day of the waving of the Omer should be forbidden in the eating of the new grain, as will be explained.
An explanation of the matter of yom hanef:
Yom hanef is the sixteenth of Nissan, the second day of Pesach, on which the Omer offering was brought. The waving is only one part of the process, but this is how the entire day came to be called. It is the bringing of the Omer that permits the eating of the new grain, and that bringing could be done at any hour of the day.
The question arises: at a time when the Beis HaMikdash does not stand, from when is the new grain permitted to be eaten? The Torah says "until this very day," meaning that until that very day arrives one may not eat of the new grain, and once the day has arrived it is permitted. From here it would follow that in the absence of a Beis HaMikdash and in the absence of an offering, it should have been permitted already from the start of the day, with sunrise.
Therefore Rabban Yochanan ben Zakkai enacted that the entire sixteenth day should be forbidden in the eating of the new grain. The reason for this is explained in the Gemara: perhaps the Beis HaMikdash will be rebuilt quickly, in a single day, and they will be able to bring the offering, and a person will assume that the grain is permitted from the start of the day, when in fact the Beis HaMikdash already stands and he must wait until the offering is brought. So that he should not eat in the morning before the offering has actually been brought, he forbade the entire day.
In summary: in this Mishnah we learned the two enactments of Rabban Yochanan ben Zakkai after the destruction: taking the lulav for seven days everywhere as a remembrance of the Beis HaMikdash, and the prohibition of eating the new grain for the whole of yom hanef, out of concern that the Beis HaMikdash will be rebuilt and people will come to eat before the Omer is offered.