פירוש על מגילה 3:6

במעמדות במעשה בראשית – Since for the sacrifices, heaven and earth were established, and the order of their readings is explained in the last chapter (4) of Tractate Taanit.

On Hanukkah they read the section of the princes (Numbers 7).
On Purim, “And Amalek came” (Exodus 17:8).
On Rosh Hodesh, “And on the first of your months” (Numbers 28:11).
On Maamadot, the account of the creation (Genesis 1:1-2:3).
On fast days, the blessings and curses (Leviticus 26:3 ff and Deuteronomy 28). They do not interrupt while reading the curses, but rather one reads them all.
On Monday and Thursday and on Shabbat at minhah they read according to the regular order and this does not count as part of the reading [for the succeeding Shabbat].
As it says, “And Moshe declared to the children of Israel the appointed seasons of the Lord” (Leviticus 23:44) it is their mitzvah that each should be read in its appropriate time.
The final mishnah of our chapter details what sections are read on non-Toraitic holidays or events. Since there are no passages in the Torah about these days, the sages had to find other passages whose themes they deemed appropriate.
The mishnah concludes with a midrash explaining why it is that we read from passages in the Torah appropriate to the holiday.
Section one: Hannukah literally means “dedication” and refers to the dedication of the Temple after its restoration in the time of the Maccabees. The portion in the Torah read on Hannukah is a list of the gifts brought by the princes of each tribe at the dedication of the Mishkah, the tabernacle.
Section two: On Purim we read about Amalek because Haman was, according to the rabbis, from Amalek.
Section four: On Ma’amadot people would gather in the Temple or in their own cities while their local kohanim took there turn at service in the Temple. See Taanit 4:2-3.
Section five: The curses (called today the “tochekhah” or rebuke) are read on fast days as a warning to people that they must repent. When reading the curses we don’t interrupt, making them into two or more aliyot rather they are all read by the same person. This is still the custom today, making one of the aliyot in Ki Tavo the longest aliyah of the year.
Section six: Besides Shabbat morning, the Torah is also read on Mondays, Thursdays and Shabbat at minhah (the afternoon service). These readings go according to the regular cycle but they don’t count toward the regular progression. This means that the same portion that is read at all three occasions and then again on Shabbat. We only move forward on Shabbat.
Section seven: The chapter ends with a midrash on Leviticus 23:44. The verse states that Moshe told the holidays to the people of Israel, but this verse is superfluous Moshe taught all of the commandments to the people. Therefore the midrash teaches that not only did Moshe teach the holidays, but he taught each one at the time that it fell. By his example we learn that on all holidays we read the Torah portion relevant to that holiday.

ברכות וקללות – “If you will follow My laws…’ (Leviticus 26:3 and following) to inform that retribution comes to the world for being engaged in sin (see specifically, Leviticus 26:14-46, “the Rebuke”) so that they will return [to God] in repentance and will be saved from their troubles.

כסדרן – according to the portion of the day

ואין עולין להם מן החשבון – so that when Shabbat arrives, they will return and read what they read on [the previous] Monday and Thursday [mornings] and on Shabbat at Minha/Afternoon Service.

שנאמר וידבר משה – this refers to the entire Mishnah, to teach from here that it is Mitzvah on each festival to read from the [subject] matter of the festival.