Because their religious holidays were celebrated on days of new or full moons, ancient Jews developed a working method of determining the monthly cycle. In ancient Jerusalem, witnesses appeared before the Sanhedrin (the Second Temple period's Jewish high court) each month to testify they had seen the new moon. Signal fires from the Mount of Olives in Jerusalem broadcast this information to the Jews in the area so they would know the new month had begun.

Passover and Rosh Hashanah