The Jewish year is a lunar one - the months begin with the new moon, and last 29 or 30 days until the next one. This is how our calendar works: a holiday which falls on the 15th of a month, such as Passover or Sukkot, will always start on the night of a full moon. Rosh Hashana, the first day of the month of Tishrei, starts with a new moon, a small sliver visible in the western sky for a short while before it sets. The start of every month, Rosh Chodesh, is celebrated as a minor holiday, with special prayers including Hallel and Musaf. There is a tradition for women to refrain from working on Rosh Chodesh, and, in modern times, the day is celebrated by some as a women's holiday.