Shemini Atzeret (שמיני עצרת - "the Eighth [day] of Assembly") is a Jewish holiday celebrated on the 22nd day of the Hebrew month of Tishrei. In the Diaspora, an additional day is celebrated, the second day being separately referred to as Simchat Torah. In Israel and Reform Judaism, the holidays of Shemini Atzeret and Simchat Torah are combined into a single day and the names are used interchangeably.

Shemini Atzeret is mistakenly referred to as the eighth day of the Festival of Sukkot, which occupies the seven preceding days. In fact, Shemini Atzeret is a holiday unto itself. There is no use of the Sukkah in Israel on Shemini Atzeret and the lulav and etrog are not waved, although, in the Diaspora, some sit in the sukkah on Shemini Atzeret; however, one of Sukkot's liturgical aliases זמן שמחתנו, "Time of Our Happiness," continues to be used to describe Shemini Atzeret in prayers.

Furthermore, the Talmud, Tractate Rosh Hashanah 4b mentions that Shemini Atzeret is a separate holiday unto itself in respect to six specific halakhic issues, but is considered the eighth day of an eight-day holiday regarding a seventh. There is a dispute amongst the commentaries regarding what those six issues are.

Since the Sukkah (and, by extension, pleasant weather) is no longer required, Jews begin to ask for rain during the Geshem prayer, which is recited in a distinctive plaintive melody during the cantor's repetition of the Musaf Amidah. In most synagogues, the cantor is clad in a kittel, a symbol of piety, owing to the vitality of a positive judgment for rain. A brief mention of rain continues to be inserted in the Amidah until Passover.

The Yizkor memorial service is also recited this day. If there is no Shabbat during the intermediate days of Sukkot, then the book of Ecclesiastes is also chanted on Shemini Atzeret.