Rosh Chodesh is the first day of every of every Jewish month.

The Jewish calandar is based on the lunar cycle, and each new month is marked by the appearance of the "new moon", i.e., a sliver of light reflected from the moon. This is typically one or two days after the so-called new moon of astronomy, when the we can only see the shadow side of the moon and not even a sliver of light.

See moon.