Shalom, everyone! Rosh Chodesh, which means “new moon,” “head of the month,” or “beginning of the month,” is the name for the first day of each month on the ecclesiastical lunar calendar for the mo’edim. Contrary to the most widely accepted teaching, the event of Rosh Chodesh was triggered by the sighting of the full moon, when the moon shines at its brightest. Rosh Chodesh is considered a minor biblical holiday, similar to the intermediate days of the holidays of Pesach (Passover) and Sukkoth (Feast of Booths).
YAH commanded B’nei Yisrael to mark the occasion of the new moon as a time of recognition. In the Book of Numbers, YAH says to Moshe (Moses):
“Also in the day of your gladness, and in your appointed seasons, and in your new moons, ye shall blow with the trumpets over your burnt-offerings, and over the sacrifices of your peace-offerings; and they shall be to you for a memorial before your God: I am the LORD your God.'” ~ Numbers 10:10
In ancient pre-exilic Israel, the occurrence of Rosh Chodesh was confirmed by the priests observing the moon. On the day after the new (full) moon sighting, a festival was held to commemorate the occasion which included a convocation, the sounding of trumpets and special sacrificial offerings. Rosh Chodesh was a very significant festival in ancient Israel. The entire calendar depended upon the declarations of Rosh Chodesh; without these declarations, there would be no way of knowing when mo’edim were supposed to occur.
As a result of undue foreign influence by their Babylonian and Persian conquerors, the post-exilic Judeans adopted the custom of observing the first sight of the waxing crescent moon as the “new moon” and beginning of a new month. They also adopted the lunisolar calendar of the Babylonians for marking time. The sighting of the waxing crescent moon then became the basis of the declarations for festivals. After the destruction of the Second Holy Temple in Jerusalem, the sighting of the waxing crescent moon was mathematically calculated using a fixed calendar developed by Rabbi Hillel II in the 4th century C.E. Over time, a custom developed whereby an additional day could be added to a month to ensure that certain holidays, for example, Yom Kippur, did not fall on days immediately before or after Shabbat (Sabbath/Seventh Day of the Week). This fixed calendar is the most widely used “Hebrew” calendar today.