Rosh Hodesh is the first day of each month (and also the last day of a thirty-day month). The command to determine the months and to sanctify the day of Rosh Chodesh according to the sighting of the new moon is learned from the verse, "This month shall be to you the beginning of months; it shall be the first month of the year to you."
The command to establish the new month according to the moon was designed to distinguish the Jewish nation from other nations of the world. The nations of the world count their years according to the sun while the Jewish people count them according to the moon. This, in turn, determines the Jewish calendar and festival, as it is written:
"And they shall be for signs and for seasons and for days and years" (Bereshit 1:14).
