The Biblical command:

The first[1] Mitzvah the Jewish people[2] were commanded in the Torah is to sanctify the new moon, as the verse[3] states “This month is to you the head of the months.”[4] This Biblical positive command obligated the Beis Din Hagadol to calculate as to when the new moon could be visible, and in the event that the new moon could be visible on the 30^th day, then they must investigate any witnesses that approach the Beis Din, and upon acceptance of the testimony they have to publicize this matter to the Jewish people. Thus, the positive command includes three stages:

1. To calculate the moon’s potential date of visibility.
2. To interrogate the witnesses.
3. To pronounce the day of Rosh Chodesh and publicize it to the Jewish people.

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[1] Rashi Bereishis 1/1; See Likkutei Sichos 26/95; This Mitzvah is the 4^th Mitzvah in Sefer Hachinuch and is the 153^rd Mitzvah in the Sefer Hamitzvos of the Rambam.

[2] This refers to the Jewish people as a nation, however our forefathers were commanded the Mitzvos of procreating, Mila, and Gid Hanashe prior to the Jewish people’s existence.

[3] Shemos 12/2

[4] In Sefer Hachinuch ibid and Sefer Hamitzvos of the Rambam [Mitzvah 153] this verse is listed as the source of this positive command. However, in Mishneh Torah Kiddush Hachodesh 1/7 the Rambam mentions the verse “Asher Tikreu Osam Mikraeiy Kodesh”.