Rosh Hashanah traditionally lasts for two days. It begins on the first day of the Hebrew month of Tishrei and continues through the second day. Although the Torah originally defines it as a one-day celebration, it is observed for two days in Orthodox and Conservative Judaism due to historical uncertainties in determining the new moon. Reform Judaism often observes only one day. The two- day period starts at sundown the evening before the first day and concludes at nightfall on the second day.