Chazal established reading four special sections of the Torah during the Purim season. The first, Shekalim, is read on the Shabbos preceding Rosh Chodesh Adar. Zachor is read on the Shabbos preceding Purim. We read HaChodesh on the Shabbos preceding Rosh Chodesh Nissan. Parah is read on the Shabbos before Parshas HaChodesh.
Each of the parshiyos commemorates an activity which occurs during this time. Shekalim commemorates the proclamation on Rosh Chodesh Adar to bring shekalim (a denomination of coin) to the Beis HaMikdash. When we read Zachor we fulfill the mitzvah of remembering what Amalek did to us. Appropriately, we read this parsha on the Shabbos before Purim since Haman descended from Amalek. Parshas Parah which describes the mitzvah of the red heifer, is a reminder to become pure before Pesach so that we can bring the korban Pesach. Finally, HaChodesh describes the mitzvah of bringing the korban Pesach.
The Torah relates that the Mishkan was built and activated on Rosh Chodesh Nissan. The first red heifer was brought only after the Mishkan was built, after Rosh Chodesh Nissan. It could not have been brought earlier because it needed the Mishkan. Since the red heifer could not have been brought beforehand, it would make more sense to read parshas Parah during the month of Nissan, when the red heifer was actually brought. Why, then, do we read Parah before parshas HaChodesh rather than following it?
The Sfas Emes explains that two critical things happened between the first Rosh Chodesh Nissan at the time of the Exodus and the second one when the Mishkan was built. At the time of the Exodus, God chose us as His nation. This is described in parshas HaChodesh which relates the first mitzvah we were given as a nation. A year later, on Rosh Chodesh Nissan, the Mishkan was activated. This second Rosh Chodesh Nissan was the eighth and final day of the initiation sacrifices of the Mishkan. With the building and activation of the Mishkan the sin of the golden calf was rectified, God’s presenced was revealed and we came close to Him, “וַֽיְהִי בַּיוֹם הַשְּׁמִינִי ... וַֽיִּקְרְבוּ כָּל־הָעֵדָה וַיַּעַמְדוּ לִפְנֵי ה'׃/It was on the eighth day … the entire community came close and stood before God.” Being chosen and coming close to God are the two key things that differentiate us as we say in the prayers of Yom Tov, “אַתָּה בְחַרְתָּנוּ מִכָּל הָעַמִים ... וְקֵרַבְתָּנוּ ... לַעַבוֹדָתֶךָ/You chose us from all the nations … and drew us near … to your service.”
At the time of the Exodus, God first chose us and then we drew near to Him. At that time we were immersed in slavery and the decadence of
Chazal allude to this at the end of last week’s parsha following the description of the initiation services and sacrifices during the eight days culminating on Rosh Chodesh Nissan, “כַּֽאֲשֶּר עָשָׂה בַּיּוֹם הַזֶּה צִוָּה ה' לַֽעֲשֹׂת לְכַפֵּר עֲלֵיכֶם׃/As he (Moshe) had done on this day, so God commanded to do in order to atone for you.” Chazal teach us that לַעֲשֹׂת/to do, refers to the red heifer. Why do Chazal find a hint to the red heifer specifically here, before Rosh Chodesh and the activation of the Mishkan? The Sfas Emes explains that Chazal are teaching this very concept. To merit being chosen by God, we must first show him that we desire to be close to Him by purifying ourselves through repentance.
The Sfas Emes says that it’s very possible that Chazal are hinting to another important idea, as well. Apparently the period immediately preceding Rosh Chodesh Nissan was designated from the time of the initiation of the Mishkan for atonement and purification. In fact, the Sfas Emes says further, that for those of us who desire and anticipate purification, an aspect of purification enters our souls before Rosh Chodesh Nissan. For this reason Chazal established reading Parah specifically prior to Rosh Chodesh Nissan. This is the time when we are given the opportunity to merit purification and coming close to God.