Rosh Chodesh

Rosh Chodesh or Rosh Hodesh (; trans. "Beginning of the Month"; lit. "Head of the Month") is the name for the first day of every month in the Hebrew calendar, marked by the birth of a new moon. It is considered a minor holiday, like the middle days of Passover and Sukkot.
When a Hebrew calendar month is 30 days long, day 30 is considered Rosh Chodesh of the next month. Then Rosh Chodesh is two days long: day 30 of the old month and day 1 of the new month.