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
