Megillah, Chapter 3, Mishnah 4. This mishnah opens the discussion of the Torah readings designated for special occasions and seasons, foremost among them the Four Parshiyos - the four special readings of the Purim and Pesach season.
Parashas Shekalim:
The mishnah opens: "Rosh Chodesh Adar shechal lihyos b'Shabbos - korin b'parashas Shekalim" - when Rosh Chodesh Adar falls on Shabbos, we read Parashas Shekalim as the maftir, together with a special haftarah. This portion is found at the beginning of Parashas Ki Sisa, and it discusses the mitzvah of giving the half shekel, which funded the communal offerings. Its reading was instituted to alert the Jewish people that by Rosh Chodesh Nissan they must contribute fresh shekalim for the coming year's offerings - an announcement and a reminder given well in advance.
And if Rosh Chodesh Adar does not fall on Shabbos but in the middle of the week - "chal lihyos b'toch haShabbos" - the law is "makdimin l'she'avar umafsikin l'Shabbos acheres": we read Parashas Shekalim early, on the Shabbos preceding Rosh Chodesh, and on the Shabbos that follows we make a break, meaning that no special maftir or haftarah is read, only the regular maftir of that Shabbos.
The reason for this: in the second week Parashas Zachor is read, in which we recall the mitzvah of wiping out Amalek, and we want it to fall on the Shabbos immediately before Purim - for Haman was a descendant of Amalek, and the two subjects should be placed next to one another. Therefore, when Parashas Shekalim is read before Rosh Chodesh Adar, a break of one Shabbos is needed so that Parashas Zachor will land on the Shabbos adjacent to Purim. But when Rosh Chodesh Adar falls on Shabbos, Parashas Shekalim is read that week, Parashas Zachor is read the following week, and Purim falls at the end of it.
The rest of the Four Parshiyos:
"Bashelishis - Parah Adumah" - in the third week we read from the beginning of Parashas Chukas, dealing with the red heifer and purification, since in the time of the Beis HaMikdash people needed to purify themselves around Purim in order to be tahor for the Pesach offering.
"Barevi'is - HaChodesh hazeh lachem" - in the fourth week we read from Parashas Bo, which contains the mitzvah of bringing the Pesach offering in preparation for the festival.
"Bachamishis - chozrin l'chesidran" - in the fifth week we return to the regular order of the weekly portions: the maftir is a repetition of the final verses of the parashah, and the haftarah is the one belonging to that parashah.
"Lakol mafsikin" - additional special readings:
"B'roshei chodashim" - when Rosh Chodesh falls on Shabbos, we read the maftir of Rosh Chodesh.
"BaChanukah" - we read the portion of the princes at the dedication of the altar, a reading that continues throughout all the days of Chanukah; and when Chanukah falls on Shabbos, this portion is read as the maftir.
"UvaPurim" - nowadays Purim never falls on Shabbos, but in the days of the mishnah the fourteenth of Adar could fall on Shabbos (even though the Megillah is not read on Shabbos), since there was no fixed calendar and the month was sanctified on the testimony of witnesses.
"Uvata'aniyos" - on a fast day that falls on Monday or Thursday we read a special reading in place of the weekly portion: from Parashas Ki Sisa, dealing with the sin of the golden calf and the forgiveness granted for it, and the revelation of the Thirteen Attributes of Mercy.
"Uvama'amados" - as we learned in Maseches Ta'anis, the men of the ma'amad would go up to Jerusalem to accompany the offerings together with the kohanim and the levi'im, or they would remain in their cities and read each day from the account of Creation. Accordingly, on Mondays and Thursdays the reading from the account of Creation took the place of the regular reading of the weekly portion.
"Uv'Yom haKippurim" - when Yom Kippur falls on Shabbos, at Minchah we read the portion of forbidden relations found in Parashas Acharei Mos, in place of the reading of the weekly portion.
In summary: in this mishnah we learned the order of the Four Parshiyos - Shekalim, Zachor, Parah and HaChodesh - and the laws of reading early and making a break, which were designed to place Parashas Zachor next to Purim, as well as the return in the fifth week to the regular order of the weekly portions. We also examined the rule of "lakol mafsikin": the special readings on Rosh Chodesh, Chanukah, Purim, fast days, ma'amados and Yom Kippur, which come either in place of the regular reading or in addition to it.