How does the Jewish calendar differ from the secular calendar?

The Jewish calendar is based on both lunar and solar cycles. It calculates the year of creation at 3761 BCE. The Jewish calendar is Israel's official calendar and all the Jewish holidays are based on it. Therefore, Rosh Hashanah lands on the first day of the Hebrew month of Tishrei every year, but it varies on the secular calendar. Each Hebrew month has 29 or 30 days, and a year has 12 lunar months plus approximately 11 days. The months start on the day of the new moon. In order to bring the calendar in line with the annual solar cycle, a 13th month of 30 days is added in the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years of a 19-year cycle. Therefore, a leap year may have from 383 to 385 days.

The secular, or "Gregorian," calendar has years of 365 or 366 days. It is divided into 12 months that have no relationship to the motion of the moon. In parallel with this system, the concept of weeks groups the days in sets of 7. There is a leap year every four years.

The secular calendar is based on Jesus' birth, which was approximately at the year zero. Every year before zero is termed Before Christ (BC). Every year after zero is termed AD for Anno Domini (the year of our lord). Jews use the terms Before Common Era (BCE) and Common Era (CE) instead, corresponding to the same years.

Hebrew Months: Tishrei, Heshvan, Kislev, Tevet, Shevat, Adar I, Adar II (added in leap years), Nisan, Iyar, Sivan, Tammuz, Av, Elul.