Feast of the New Moon
A new Moon is the day on which the earliest visible increasing crescent appears, which signals the start of a new month. The new moon signifies the start of every Jewish month and is considered an important date and minor holiday in the Hebrew calendar.
“…on the day of your joy and at your appointed feasts, and on the first days of your months, you shall blow the trumpets over your burnt offerings, and over the sacrifices of your peace offerings; and they shall be as a reminder of you before your God. I am the Lord your God.” —Numbers 10:10 NASB
“Blow the trumpet at the new moon,
At the full moon, on our feast day.” —Psalm 81:3 NASB
The name for the first day of every month in the Hebrew calendar is Rosh Chodesh or Rosh Hodesh (Hebrew: ראש חודש). Rosh Chodesh is the day after a new moon. The occurrence of Rosh Chodesh was originally confirmed on the testimony of witnesses observing the new moon. After the Sanhedrin declared Rosh Chodesh for either a full month or a defective, 29-day month, news of it would then be communicated throughout Israel.