Because Hebrew calendar days begin at sundown, Rosh Hashanah starts at sundown.
The holiday begins on the evening before 1 Tishrei, at the close of 29 Elul. This follows the biblical and Jewish calendrical pattern in which a day runs from evening to evening, rather than from midnight to midnight.
Rosh Hashanah is commonly called the Jewish New Year, but it is also the first day of Tishrei and the opening of the High Holy Days. Its observance includes synagogue prayer, reflection, and hearing the shofar. The timing of the actual start varies by location because sundown differs from place to place.
A common mix-up is confusing the holiday’s religious start with the civil-calendar date printed on a modern calendar. Jewish holidays generally begin the preceding evening; Rosh Hashanah is no exception. In traditional practice, the festival is observed for two days, though the Torah describes it as a one-day celebration.