THE HEBREW CALENDAR

Months (in a "regular" year):

1. Tishrei (Tishri) - 30 days
2. Chesvan (Heshvan) - 29 days*
3. Kislev - 30 days**
4. Tevet - 29 days
5. Shvat (Shevet) - 30 days
6. Adar I - 30 days***
7. Adar II - 29 days
8. Nisan - 30 days
9. Iyyar (Iyar) - 29 days
10. Sivan - 30 days
11. Tamuz (Tammuz) - 29 days
12. Av - 30 days
13. Elul - 29 days

*In a "complete" year, Chesvan has 30 days.
** In a "deficient" year, Kislev has 29 days.
***Adar I is added in leap-years. In non-leap-years, Adar II is simply called Adar. A year is a leap-year if the year number divided by 19 is equal to 0, 3, 6, 8, 11, 14, or 17.

Explaining what years are "deficient", "regular", and "complete" is rather complex. I may get to it, but until then, just read sections 3.3 - 3.8 of The Calendar FAQ.