The Month
The Moon passes through her different phases in 29 days, 12 hours, and 793 parts (halakim) of an hour.
Ps 104:19 "He appointed the moon for seasons; The sun knows its going down."
These phases of the moon serve as a measure of time and their period is one lunar month. In simplicity the months are determined by full days and set with the beginning of night. Each month contains either 29 or 30 days.
The first appearance of the new moon determines the beginning of the month. It is first noticed as a small and faded arc, like a sickle, that is barely visible. Certain people with excellent eyesight would be the first to see it, yet to avoid confusion the announcement was made by the high priest who determined the New-Moon Day for the whole nation. A fire would be lit upon the Mount of Olives, which would then signal messengers waiting upon the surrounding hills, even beyond the boundaries of Israel to the Diaspora. Sometimes, as in the case of war, the revelation that the new moon had appeared over Jerusalem was sent by a written message. This helped to preserve the religious unity of the nation, and insure the uniform celebration of "the seasons of the Lord" throughout the world.
The Jewish Calendar in Ancient Hebrew History