The Magen David Series - Rosh Chodesh
In Hebrew, Rosh Chodesh means, literally, “head of the month” or “first of the month.” Rosh Chodesh is the first day of any new month. If a month is 30 days long, then the 30th day is treated as part of the Rosh Chodesh for the next month, and the Rosh Chodesh for next month extends for two days (the 30th of the earlier month and the 1st of the later month).
In ancient times, Rosh Chodesh was a significant festival day. At that time, the new months were determined by observation. Each month began when the first sliver of moon became visible after the dark of the moon. Observers would watch the sky at night for any sign of the moon. If they saw the moon, they would report their sightings to the Sanhedrin, which would interrogate them to make sure that they were not mistaken.