Shemini Atzeret (שְׁמִינִי עֲצֶרֶת—"Eighth [day] of Assembly") is a Jewish holiday. It is celebrated on the 22nd day of the Hebrew month of Tishrei, which usually coincides with late September or early October. It directly follows the festival of Sukkot, which is celebrated for seven days; thus, Shemini Atzeret is literally the eighth day [of assembly]. It is a separate—yet connected—holy day devoted to the spiritual aspects of the festival of Sukkot. Part of its duality as a holy day is that it is simultaneously considered connected to Sukkot and a separate festival in its own right.

Outside the Land of Israel, this is further complicated by the additional day added to all biblical holidays except Rosh Hashanah and Yom Kippur. Shemini Atzeret is thus sometimes wrongly regarded as the eighth day of Sukkot outside the Land of Israel, leading to sometimes involved analysis as to which practices of each holiday are to apply.

The celebration of Simchat Torah is the most distinctive feature of the holiday, but it is a later rabbinical innovation. In the Land of Israel, the celebrations of Shemini Atzeret and Simchat Torah are combined on a single day, and the names are used interchangeably. In the Diaspora, the celebration of Simchat Torah is deferred to the second day of the holiday. Commonly, only the first day is referred to as Shemini Atzeret, while the second is called Simchat Torah.

Karaite Jews and Samaritans also observe Shemini Atzeret, as they do all biblical holidays. However, due to differences in calendar calculations, it may occur on a different day from the conventional Jewish celebration. Karaites and Samaritans do not include the rabbinical innovation of Simchat Torah in their observance of the day and do not observe a second day—of any holiday—in the Diaspora.

According to The Jewish Encyclopedia, atzeret (Biblical Hebrew: עצרת, romanized: ʿaṣereṯ, lit. 'assembly') is the name given to this day in four different locations in the Hebrew Bible. It is not mentioned in Deuteronomy 16, and is found only in those parts of the Bible known as the Priestly Code. Like atzarah, atzeret denotes "day of assembly", from atzar ("to hold back" or "keep in"); hence the name atzeret given to the seventh day of Pesaḥ. Owing, however, to the fact that both Shemini Atzeret and the seventh day of Pesaḥ are described as atzeret, the name was taken to mean "the closing festival".

When Shemini Atzeret is mentioned in the Torah (known in Greek as the Pentateuch), it is always mentioned in the context of the seven-day festival of Sukkot: the holiday Shemini Atzeret immediately follows. For example, Sukkot is described in detail in Leviticus 23:33–43. Shemini Atzeret is mentioned in only verses 36 and 39.

The Hebrew word shemini means eighth. This refers to the date of Shemini Atzeret relative to Sukkot; it falls on the latter's eighth day. It is often assumed that Shemini Atzeret is simply the eighth day of Sukkot. That characterization, however, is only partly accurate.

The celebration of Sukkot is characterized by the use of the sukkah (booth or tabernacle) and the Four Species (tree branches and fruit used in the celebration). However, the Torah specifies using those objects for only seven days, not eight. The observance of Shemini Atzeret, therefore, differs in substantial ways from that of Sukkot. The Talmud describes Shemini Atzeret with the words "a holiday in its own right" (regel bifnei atzmo).