The number of days of Rosh Chodesh depends on whether the month is twenty nine days long or thirty days. If it is thirty days the thirtieth day will contain the actual "birth" of the new moon and it is considered to be the first day of Rosh Chodesh, and the first day of the new month is the second day of Rosh Chodesh. When the month is only twenty nine days long the new moon is "born" on the next day, the first day of the new month, and that first day of the month is the only day of Rosh Chodesh.
All the months with the exceptions of Cheshvan and Kislev have the same number of days every year.
- Tishrei - 30 days
Cheshvan - 29 or 30 days
Kislev - 29 or 30 days
Tevet - 29 days
Shvat - 30 days
Adar - 29 days (when there are two months of Adar the first one always has 30 days)
Nisan - 30 days
Iyar - 29 days
Sivan - 30 days
Tamuz - 29 days
Av - 30 days
Elul - 29 days
