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How does the Jewish calendar differ from the Gregorian calendar?

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Circular diagram of the Jewish religious year showing Hebrew months around the outer ring and Gregorian months inside, with major holidays and fasts such as Rosh Hashanah, Yom Kippur, Sukkot, Hanukkah, Passover, and Purim labeled with their dates. — Jewish religious year The months and holidays of the Jewish religious year.

The Jewish calendar differs from the Gregorian calendar primarily in its structure and method of tracking time. While the Gregorian calendar is a solar dating system, the Jewish calendar is lunisolar, meaning it is regulated by the positions of both the Moon and the Sun.12

Key differences include:

Year Length: A standard Jewish year consists of 12 lunar months, totaling 353, 354, or 355 days. Because this is shorter than the solar year, the calendar periodically adds a 13th month (a 30-day leap month) seven times every 19 years—a system known as the Metonic cycle—to ensure festivals remain in their proper seasons. In contrast, the Gregorian calendar is solar and adds a single leap day every four years, with specific exceptions for century years.132
Variability: Due to its lunisolar structure, the number of days in a Jewish year varies significantly (from 353 to 385 days), and the first day of a month can fall on any day of the week, changing from year to year.12
Era: The Jewish calendar counts years from the anno mundi (the era of Creation), which began in 3761 BCE.3