Rosh Chodesh: (Hebrew: ראש חודש; trans. Beginning of the Month; lit. Head of the Month) is the name for the first day of every month in the Hebrew calendar, marked by the appearance of the new moon. Contrasted with the astronomical definition of new moon – which is not visible to the naked eye – the new moon in the Hebrew calendar is marked by the day and hour that the new crescent is observed.
Despite the existence of a fixed calendar, Rosh Chodesh is still announced in synagogues on the preceding Shabbat (called Shabbat Mevarchim—The Shabbat of Blessing [the new month]). The announcement is made after the reading of the sefer torah, before returning it to the aron kodesh, in a prayer beginning “May it be Your will… that You renew this month for us for good and for blessing.” The name of the new month, and the day of the week on which it falls, is given during the prayer.