jewishcalendar

Name	Number	Length in a Deficient Year	Length in a Regular Year	Length in a Complete Year	Gregorian Equivalent	Important Feast Days
Nisan	1 or 7	30 days	30 days	30 days	March-April	Passover and Feast of Unleavened Bread
Iyar	2 or 8	29 days	29 days	29 days	April-May
Sivan	3 or 9	30 days	30 days	30 days	May-June	Feast of Pentecost
Tammuz	4 or 10	29 days	29 days	29 days	June-July
Av	5 or 11	30 day	30 day	30 day	July-August
Elul	6 or 12	29 days	29 days	29 days	August-September
Tishri	7 or 1	30 days	30 days	30 days	September-October	Feast of Trumpets^(a) Feast of Ingathering
Cheshvan (b)	8 or 2	29 days	29 days	30 days	October-November
Kislev(b)	9 or 3	29 days	30 days	30 days	November-December
Tevet	10 or 4	29 days	29 days	29 days	December-January
Shevat	11 or 5	30 days	30 days	30 days	January-February
Adar(c)	12 or 6	29 or 30 days	29 or 30 days	29 or 30 days	February-March
Adar II(d)	13	29 days	29 days	29 days	February-March

Trumpets, Feast of: was celebrated at the beginning of the month Tishri, the first month of the civil year. It received its name from the circumstances that the trumpets usually blown at the commencement of each month were on that occasion blown with unusual solemnity (Lev. 23:23-25; Num. 10:10; 29:1-6). It was one of the seven days of holy convocation.
The length of Cheshvan and Kislev are determined by complex calculations involving the time of day of the full moon of the following year's Tishri and the day of the week that Tishri would occur in the following year. There are plenty of easily accessible computer programs that will calculate the Jewish calendar for more than a millennium to come (Download Page see Fourmilab's Hebrew calendar converter).
In leap years, Adar has 30 days. In non-leap years, Adar has 29 days.
Adar II: Also called "Adar Sheni," or "Veadar," this month of 29 days is added about once every three years in order to adjust the Jewish calendar of 354 days to the solar calendar. Adar II falls between Adar and Nisan in late February and early March.

Note: The number of days between Nissan and Tishri is always the same. Because of this, the time from the first major festival (Passover in Nissan) to the last major festival (Ingathering in Tishri) is always the same.