Rosh Chodesh - The New Month
The first day of each month and the thirtieth day of the preceding month, if it has thirty days, is (in modern times) a minor holiday known as Rosh Chodesh (head of the month). The one exception is the month of Tishrei, whose beginning is a major holiday, Rosh HaShanah. There are also special prayers said upon observing the new Moon for the first time each month.
(Adapted from Wikipedia)