Which calendar is used for the New Year festival called Rosh Hashanah in Judaism?

Rosh Hashanah is a Jewish New Year festival determined by the Hebrew calendar.

The Hebrew calendar is a lunisolar system: its months follow lunar cycles, while adjustments keep the festivals aligned with the solar year. Rosh Hashanah begins on the first day of Tishrei, the seventh month of the religious year, and generally falls in September or October on the Gregorian calendar.

The festival is observed over one or two days, depending on Jewish tradition and location. Customs include sounding the shofar, eating symbolic foods such as apples dipped in honey, and beginning a period of reflection that leads toward Yom Kippur. A common misunderstanding is that Rosh Hashanah always falls on January 1; it does not, because it follows the Hebrew rather than the Gregorian calendar.