Rosh Chodesh dates

Why does Rosh Chodesh vary between one and two days? For example this year Nisan is one day, Iyar is two days, Sivan is one and Tammuz is two. It seems to be that every other month is one day and the next is two days, but why? Morris Altschuler

Hi Morris,

The Jewish calendar is primarily based the revolution of the moon about the Earth. On average, the moon revolves around the Earth in approximately 29½ days (to be more precise, 29.53059 days).

Since a month needs to consist of complete days, a Jewish calendar month will generally alternate between 29 or 30 days, corresponding to the 29½-day lunar cycle.

A month that is 29 days long is known as chaser, “missing”, while a month that is 30 days long is known as malei, “full”.

A month that has only 29 days, will be followed by ONE day of Rosh Chodesh, which is considered the first day of the new month.

When a month has 30 days, the 30th day of the (old) month will be observed as the first day of Rosh Chodesh of the following month, and the 2nd day of Rosh Chodesh is considered the first calendar day of the (new) following month.