“And the LORD spoke unto Moses and Aaron in the land of Egypt, saying: ‘This month shall mark for you the beginning of the months; it shall be the first of the months of the year for you.'” (12:1-2)
In the Book of Numbers, God speaks of the celebration of the new moon to Moses:
“And on your joyous occasions – your fixed festivals and new moon days – you shall sound the trumpets over your burnt offerings and your sacrifices of well-being.” (10:10)
In Psalm 81:3, both new and full moon are mentioned as a time of recognition by the Hebrews:
“Blow the trumpet at the time of the New Moon, at the full moon, on our solemn feast day. For this is a statute for Israel, a law of the God of Jacob.
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 and the diaspora.