The first mitzvah given to the Jews after they became a nation was to determine and sanctify the first day of every month—Rosh Chodesh—thereby creating the Jewish calendar. This command is expressed in the verse cited above, הַ ח ֹ דֶ שׁ הַ זּ ֶ ה ל ָ כֶ ם רֹאשׁ חֳ דָ שִׁ ים. The words החדש הזה can also be translated as “this renewal,” meaning that G‑d showed Moshe the crescent moon and said, “This renewal—when the moon renews itself—shall be [what determines] for you the heads of the months.”
The priority given to this mitzvah suggests that sanctifying Rosh Chodesh is a model mitzvah, representing the underlying theme of all the other mitzvos.
The primary objective of all mitzvos is to transform the physical world from mundane to holy. By using any physical object to perform a mitzvah, we reveal the G‑dly purpose for which that object was created, thereby sanctifying said item. The sanctification of Rosh Chodesh embodies this idea, for in this mitzvah time itself is elevated. Namely, this mitzvah involves taking a day that was previously like any other, and declaring it Rosh Chodesh—no longer a regular weekday, but a day replete with special offerings brought in the Beis Hamikdash. In addition, setting up the calendar requires the Beis Din to calculate the constant cycles and patterns of the sun and moon. Thus, the mitzvah to establish a Jewish calendar not only elevates the days sanctified as Rosh Chodesh (and by extension, the holidays observed on specific dates within the months), it reveals the G‑dly purpose within the entire passage of time.
Rosh Chodesh was therefore the first mitzvah commanded, since it is a visible act of sanctifying the mundane—the essential theme of all the mitzvos. Moreover, time, which marks and is defined by change, is the first and most basic characteristic of every created being: the change from non‑existence to existence. As such, just as time is the very first creation, its sanctification is the very first mitzvah.
—Likkutei Sichos, vol. 26, pp. 61–65
