Question: Why do we read four special Torah sections between Purim and Pesach. Also, why do we call each of the four Shabbatot on which we read these sections by a special name – such as Shabbat Shekalim, Shabbat Zachor etc.?
Answer: The four sections you refer to – the arba parshiyot – are read starting from the Shabbat preceding the first of Adar through the Shabbat preceding the first of Nissan. This year, the 25th of Shevat, Shabbat Parshat Mishpatim, was Shabbat Shekalim; Shabbat Parshat Tetzaveh on the 9th of Adar was Shabbat Zachor; the 23rd of Adar, Shabbat Parshat Vayakhel-Pekudei, was Shabbat Parah; and Rosh Chodesh Nissan, Shabbat Parshat Vayikra, will be Shabbat HaChodesh.
There are additional Shabbatot referred to by special names, such as Shabbat Nachamu after Tisha B’Av; Shabbat Shuva (also known as Shabbat Teshuvah) between Rosh Hashanah and Yom Kippur; Shabbat Shira, the name given to the Shabbat on which we read Parshat Beshalach; and Shabbat Hagadol, which is always the Shabbat preceding Pesach.
On these Shabbatot we do not read a special Torah section in addition to the parshat ha’shavua, but their names denote a significant factor distinguishing them from a “regular” Shabbat. For example, Shabbat Shira is named such because it the Shabbat on which we read the Shirat Hayam – the songs of praise sung at the Red Sea by the Jewish people led by Moses and Miriam.
And now to the arba parshiyot. The gaon, Rabbi Shlomo Yosef Zevin, zt”l, writes as follows in his HaMoadim BaHalacha (Jerusalem, 1956, p. 188): “The unique importance of these parshiyot is found in rabbinic literature. Even before the parshiyot of the Torah were divided among the Shabbatot of the year, the requirement [to read] the arba parshiyot was already noted [Megillah 29a]. However, in both the Mishnah and the Tosefta we find no mention of the names of the weekly parshiyot [nor the requirement to read parshat ha’shavua].
“The requirement to read the Torah every Shabbat (in a congregation of ten) actually dates back to the time of Moses [Bava Kamma 82a].”
This Gemara enumerates 10 enactments of the prophet Ezra. Among them is that the Torah be read publicly every Shabbat at Mincha, as well as on Monday and Thursday. The Gemara questions this list of 10, arguing that the enactment to read the Torah on Shabbat, Monday, and Thursday dates back to Moses.
The Gemara proves (or roots) its point based on two verses. Exodus 15:22 states, “Moses brought the Children of Israel from the Red Sea and they went out toward the Desert of Shur; they traveled three days in the desert without finding water.” And Isaiah 55:1 states “Everyone who is thirsty, go for water.” “Water” is often used metaphorically to refer to Torah. The verse makes evident that going for three days without water is not desirable and, hence, we learn that we should read the Torah, i.e. “water,” thrice weekly so that three days never pass without Torah.
In light of this point, the Gemara backtracks and clarifies that reading from the Torah began with Moses and states that Ezra’s enactment was merely an “upgrade” – Moses required that three verses be read while Ezra enacted that we must read 10 verses and call up three people to the Torah, a kohen, a levi, and yisrael.
Rabbi Zevin continues: “Dividing the Torah reading into 54 [weekly] parshiyot came at a much later time [than Ezra]. We find (in Megillah 29b) that the bnei ma’arava in Eretz Yisrael, [as opposed to the Jews in Babylonia] used to conclude reading the entire Torah every three years.”
The Mechaber (Shulchan Aruch, Orach Chayim 669:1) clearly states that on the second day of Shemini Atzeret, we in the Diaspora read Vezot HaBeracha, the last parshah of the Torah, until its conclusion and then begin reading Parshat Bereishit until “asher bara Elokim la’asot.” (We then read, for maftir, “Bayom hashemini atzeret.”)
Since the Gemara states that the bnei ma’arava would complete the Torah once every three years, we might infer that the Jews in exile in Babylonia completed the Torah every year. The Gemara does not state this specifically, but we can derive it from two statements in the Gemara. First, R. Yirmiyah (Megillah 30b) rules that the four parshiyot’s Torah readings only cause a change in the haftarah; on the succeeding Shabbat we resume reading the usual weekly haftarah (see Rashi sv. “l’seder haftarot…”). Second, the Gemara (ibid., 31a) states that on the last day of Shemini Atzeret (Simchat Torah in the diaspora) congregations would read Vezot HaBeracha.
Thus we have answered your second question first. The arba parshiyot and the times we read them are clearly mentioned in the Mishnah (Megillah 29a) by name. Therefore the Shabbatot on which one of them is read are referred to by its name.