The Hebrew Calendar is based on the lunar calendar and consists of a 19 year Cycle. Leap Years occur in a fixed arrangement called the Metonic Cycle The 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of this 19 year Lunar Cycle are leap years. Each Leap Year in a Lunar Cycle has one extra month. It is called Adar II.
The Hebrew Calendar was changed in the fourth century
by Hillel II. Before the change the month Elul could have
30 days based on the observation of witnesses. If the
new moon was seen on Elul 30, it became Tishri 1. Today
Rosh Ha Shanah is celebrated on Tishri 1 and 2.
Hebrew Calendar compared to the keyboard on a Piano
The Historical VS the Religious Calendar