In his first comment, Rashi opens his study of Chumash by repeating a medrash that asks why the Torah start with creation rather than with the first commandment to the Jewish People:

הַחֹ֧דֶשׁ הַזֶּ֛ה לָכֶ֖ם רֹ֣אשׁ חֳדָשִׁ֑ים רִאשׁ֥וֹן הוּא֙ לָכֶ֔ם לְחָדְשֵׁ֖י הַשָּׁנָֽה׃

This month [this stage of the moon (the L-rd “pointing it out” to Moses)] [Nissan] shall be to you the beginning of months. It is the first for you of the months of the year.

– Shemos 12:2
tr. R’ Shraga Silverstein, with insertions as per Rashi’s commentary

And Rashi (still quoting the Medrash) answers, that it’s to justify morally our claim to Israel — we were given it by the Manufacturer.

But then there is the opposite question… Of all the mitzvos given to the Jewish People as a whole, why is this one the first one given?

One possibility is in the word chodesh, The term for “month” here really refers to the “new” in “new moon”. For that matter, shanah, year, refers to repetition and age, in contrast to that new month. So, if one took an alternate hyper-literal approach to the pasuq, it would be read:

This new-thing shall be be for you the head of the new; it is the first of the newnesses of the reiteration.

Anyone who is thinking in the language the Torah would find it very fitting that the halachic content of the Torah begins with this mitzvah, all about humanity declaring the new, the start.

On Shabbos, the berakhah in the Amidah special for the day closes, “מקדש השבת — Who sanctifies the Shabbos.” But one parallel berakhah for Yom Tov ends “מקדש ישראל והזמנים — Who sanctifies Israel and the times”. If there is any relationship between the sanctity of Shabbos and that of the Jewish People, our holiness flows from that of Shabbos. But when it comes to the holidays, their date depends  on which days we make rosh chodesh — the beginning of the month.  Their holiness derives from ours.

I was taught early in grade school that fulfilling the qorban Pesach that first Pesach required a certain bit of nerve, because the Egyptians held sheep to be sacred, and here we were, trying up lambs for four days and preparing to offer them as sacrifices. Thus, those who were willing to do this rite, and put their lamb’s blood on their doorpost truly proved their separation from Egyptian culture. This idea is also utilized to explain Bereishis 46:34:

וַאֲמַרְתֶּ֗ם אַנְשֵׁ֨י מִקְנֶ֜ה הָי֤וּ עֲבָדֶ֙יךָ֙ מִנְּעוּרֵ֣ינוּ וְעַד־עַ֔תָּה גַּם־אֲנַ֖חְנוּ גַּם־אֲבֹתֵ֑ינוּ בַּעֲב֗וּר תֵּשְׁבוּ֙ בְּאֶ֣רֶץ גֹּ֔שֶׁן כִּֽי־תוֹעֲבַ֥ת מִצְרַ֖יִם כָּל־רֹ֥עֵה צֹֽאן׃

Then you shall say: Your servants have been men of the flock from our youths until now, both we and our fathers. (Say this) so that you be settled in the land of Goshen, [which you require for pasture. And he will settle you there, away from him,] for the abomination of Egypt are all who graze sheep, [sheep being an object of idolatry to them,]

However, as the Avudraham notes, there is no evidence of sheep actually being revered in Ancient Egypt. He suggests instead that the issue is the Zodiac — Egyptians had a problem with shepherds because they would perforce be leading their flocks during Aries (the sign of the lamb, the month of Nissan). The qorban Pesach was thus chosen to be the animal that was that month’s zodiacal sign.

The idea that Hashem gave humans control of the calendar differs greatly from the view of time that lies behind astrology. Astrology sees man as subject to metaphysical forces that flow through the stars and planets. And since those run in predetermined cycles, it is a view of a humanity that are victims of fate. (“Mazal Tov!” has more to do with predetermination and predictability than wishing someone “Good Luck!”) The mitzvah to create our own calendar says that we decide the months, when the month of Adar, that of Pisces (fish), ends, and when Nissan, Aries (the ram) begins. A humanity that creates our own destiny. “This month, this newness, this power to innovate, shall be for you…”

The notion that people can start causal chains rather than being leaves blown by the winds, our power to truly initiate something new, to be in the “image” of the Creator by ourselves creating, seems a very fitting start to halakhah.