Tishri (30 days)
Tebeth (29 days)
Shebat (30 days)
Adar (29 days)
Nissan (30 days)
Iyyar (29 days)
Sivan (30 days)
Tammuz (29 days)
Ab (30 days)
Ellul (29 days)
The Hebrew, or Jewish, calendar developed over a long period and is not an old one in its present form. The original calendar is believed to have been primarily lunar and was probably based on observations rather than calculations.
During the 7th century B.C. (or C.E., "Common Era"), intercalary months were applied to adjust the lunar year to the solar year, but these were used now and then, and it was not until sometime during the 4th century A.D. that the calendar became "fixed".
During the Babylonian exile in the 6th century B.C., the Jewish calendar was greatly influenced by the Babylonian system. For example, the Jewish months were designated by numbers or with agricultural references.
Then as their months began to be called by the Babylonian names, the Hebrew names disappeared. The New Year, which had been celebrated in Nissan, was changed to Tishri, which is when the Babylonians started the beginning of their years.
The present Jewish calendar is luni-solar with the years being reckoned by the sun and the months by the moon. The day is designated as starting at 6 p.m. for the calendar. For religious and practical purposes, it begins at sunset.
Religious limitations, or specifications, complicate the Jewish calendar by dictating that certain events may not occur on certain days: The New Year must not fall on a Sunday, Wednesday or Friday; the Day of Atonement must not fall on a Friday or Sunday; the Day of the Tabernacle must not fall on a Saturday; Passover must precede the New Year by 163 days, and Pentecost must precede the New Year by 113 days.
To accommodate these specifications, a system that makes the years variable in length is used. The month is calculated from conjunction to conjunction, which is equal to 29 days, 12 hours, 44 minutes, and 3.33 seconds.
To make sure that they have an exact number of days, a system of alternating 29 and 30 days came into existence. The months which contain 30 days are called "full" and those that have 29 days are called "defective". Most of the months have fixed lengths, but there are two which are variable.