Which calendar system sets the date of Rosh Hashanah, the Jewish New Year?

Rosh Hashanah, the Jewish New Year, is dated according to the Hebrew calendar.

The Hebrew calendar is lunisolar: its months follow lunar cycles, while adjustments keep the calendar aligned with the solar seasons. Rosh Hashanah begins on the first day of Tishrei, the seventh month in the religious ordering of the Hebrew Bible. In the civil reckoning of the Hebrew calendar, Tishrei is the first month of the year.

Because the Hebrew calendar does not match the Gregorian calendar, Rosh Hashanah falls in September or October. The holiday begins at sunset and lasts two days in many communities, with prayer, festive meals, and the sounding of the shofar, a ram’s-horn trumpet.

Rosh Hashanah is not simply a secular countdown celebration. It begins the Ten Days of Repentance, leading to Yom Kippur. The Hebrew calendar also determines other Jewish holidays, including Passover and Sukkot, through its months and intercalary leap years.