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קורין פרשת שקלים – [The Torah portion of] Kee Tissa (Exodus 30:11-16), to announce that [people] should bring their Shekels in Adar in order that they can make offerings on the first of Nisan from new Terumah/heave-offerings.

If Rosh Hodesh Adar falls on Shabbat the portion of shekalim is read [on that day]. If it falls in the middle of the week, it is read on the Shabbat before, and on the next Shabbat there is a break.
On the second [of the special Shabbatot] they read “Zakhor;”
On the third the portion of the red heifer;
On the fourth “This month shall be for you;”
On the fifth the regular order is resumed.
They interrupt [the regular order] for anything: for Rosh Hodesh, for Hanukkah, for Purim, for fasts, for Ma’amadot, and for Yom HaKippurim.
The rest of this chapter deals with the portions of the Torah read on holidays and special Shabbatot. In mishnaic times they did not complete the Torah once a year as they did in Babylonia and as we do today, but rather about once every three years. Another difference between the ancient custom and that of today is that today when certain holidays fall on Shabbat we read the regular Torah portion and then we add a special reading for that day. In mishnaic times, since they didn’t really have a regular Torah portion, they only read the special reading. Thus if Rosh Hodesh fell on Shabbat they would read only the portion for Rosh Hodesh and interrupt the regular continuous reading of the Torah.
Our mishnah deals with the four special Shabbatot that precede Pesah. They are:
1) Shekalim to remind people that on Adar they would have to bring the half-shekel to the Temple (see tractate Shekalim). This was read before Rosh Hodesh Adar.
2) Zakhor Deuteronomy 25:17-19. This is read before Purim and connects Amalek with Haman.
3) Parah Numbers 19. We read about the red heifer to remind people that before Pesah they must be pure in order to eat the Pesah sacrifice.
4) Hahodesh Exodus 12:1-20. Read the Shabbat before Nissan to remind people that Pesah is approaching and that they must begin preparing.
Section one: On Rosh Hodesh Adar which falls on Shabbat they read Shekalim, which is Exodus 30:11-16. However, if Rosh Hodesh Adar falls during the week, they would read Shekalim on the Shabbat before Rosh Hodesh. On the Shabbat following Rosh Hodesh they would go back to reading where they had last left off in the regular cycle. This week would then be a break from the four special portions enumerated in our mishnah.
Sections 2-5: The mishnah now enumerates the four special portions, described above in the introduction. After Hahodesh, the order returns to its regular cycle. We should note the concept of Shabbat Hagadol, the Shabbat before Pesah, did not exist in mishnaic or talmudic times.
Section six: The regular reading of the Torah is interrupted for any special occasion. This includes all holidays. On fast days, meaning Mondays or Thursdays when they would fast for rain, they would not read the regular portion but rather the special readings for fasts. Ma’amadot were described in greater length in tractate Taanit. These were gatherings by people in towns when their kohanim would go to the Temple. The people in the town would read from the beginning of Genesis and not from the regular Torah portion.

מקדימין לשעבר – the read in the portion of Shekalim (Exodus 30:11-16) on the Sabbath that is before Rosh Hodesh, and even if Rosh Hodesh Adar falls on Friday, we advance the reading to the previous Shabbat.

ומפסיקין בשבת הבאה – from reciting the second portion, in order that the portion of Zakhor (Deuteronomy 25:17-19) would be read on the Sabbath closest to Purim, to juxtapose the wiping out/destroying of Amalek with the wiping out/destroying of Haman.

בשלישית פרה אדומה – to warn the Jewish people to purify themselves and that they should make their Passover sacrifices in purity. And which is the third Sabbath? That which is closest to immediately after Purim. And when Rosh Hodesh Nisan occurs on Shabbat, it would be that the third Shabbat is the Shabbat closest to Rosh Hodesh Nisan, in order to juxtapose the warning of the purification of those who became defiled [through contact] with the dead to the Passover [offerings].

ברביעית החדש הזה – for there is the portion of Passover (Exodus 12:1-20).

לכסדרן – to the order of the Haftarot/prophetic portions, for until here, we recite Haftarot that are an abstract of/a reflection of the four [special] Torah portions: The portion of Shekalim – “At years of age” (II Kings 12:1); the portion of Zakhor: “…I am exacting the penalty for what Amalek did to Israel…”(I Samuel 15:2 – though many customs today have the community read the entire chapter); the portion of Parah: “And I will sprinkle upon you purifying waters” (Ezekiel 36:15); the portion of HaHodesh: “On the first day of the first month” (Ezekiel 45:18). From here and onwards, he returns to recite the prophetic portion that is a reflection of the [Torah] portion of that day.

לכל מפסיקין – We do not recite a prophetic portion that is a reflection of that day’s [Torah] portion, but rather, an abstract of the event.