Jewish Calendar – A “day” begins and ends at sunset. It is based on the rotation of the Earth about its axis (a day); the revolution of the moon about the Earth (a month); and the revolution of the Earth about the sun (a year). These three phenomena are independent of each other.
The moon revolves around the Earth in about 29½ days
The Earth revolves around the sun in about 365¼ days, that is, about 12.4 lunar months.
The lunar month on the Jewish calendar begins when the first sliver of moon becomes visible after the dark of the moon.
The problem with a strictly lunar calendar is that there are approximately 12.4 lunar months in every solar year, so a 12-month lunar calendar is about 11 days shorter than a solar year and a 13-month lunar is about 19 longer than a solar year. To compensate for this drift, the Jewish calendar uses a 12-month lunar calendar with an extra month occasionally added. The month of Nissan occurs 11 days earlier each year for two or three years, and then jumps forward 30 days, balancing out the drift.
In ancient times, this month was added by when the Sanhedrin observed the conditions of the weather, the crops and the livestock, and if these were not sufficiently advanced to be considered “spring,” then the Sanhedrin inserted an additional month into the calendar to make sure that Pesach (Passover) would occur in the spring.
The current date on the Jewish calendar represents the number of years since Creation. This number is determined by adding all the ages of people mentioned in the Bible back to the first day. Many Jews, from Reform to Orthodox, accept the idea that the first six “days” of creation may not have been the 24-hour days we recognize today.
Months of the Jewish calendar are referred to mostly by number in the Bible, but were given names almost identical to the names for the Babylonian month.
Tevet – Jan
Shevat – Jan – Feb
Adar I – Feb – Mar * Last month of the Jewish year – a time of joy
(Adar II) – Occurs in a leap year
Nissan – Mar – April * Beginning of the Jewish year 5771
Iyar – April – May
Sivan – May – June
Tamuz – June – July
Av – July – Aug
Elul – Aug – Sept
Tishrei – Sept – Oct
Cheshvan – Oct – Nov
Kislev – Nov – Dec