Rosh Hashanah is traditionally a two-day Jewish holiday. It begins at sundown on the first day of Tishrei (the seventh month in the Hebrew calendar) and lasts through the end of the second day. This two-day observance originated from an ancient practice to ensure the correct day was celebrated due to difficulties in determining the lunar calendar date. The two days together are considered "one long day" with full observance on both days, making the holiday last approximately 48 hours.
