Shemini Atzeret (Hebrew שְׁמִינִי עֲצֶרֶת “Eighth [day of] Assembly”) is an annual Jewish holiday that marks the eighth day of Sukkot in the global diaspora. It is both connected to and yet separate from Sukkot. It is celebrated on the 22nd day of Tishrei; the rabbinical second day celebrated on the 23rd day of Tishrei, known as Simchat Torah, is commonly observed as a separate holiday and is treated as such by the Fellowship. In 2021 Shemeni Atzeret will be celebrated from the evening of September 27th to the evening of September 28th.
Mentioned in Leviticus, Shemini Atzeret is always discussed in the context of Sukkot, which it immediately follows. The Talmud describes Shemini Atzeret as both the end holiday of the Festival of Sukkot and as a holiday in its own right. Although it is a bonafide holiday it is also coterminous with the eighth day of Sukkot in the global diaspora. Shemini Atzeret contains the root atzor which can mean stop, tarry, or retain; that root implies that the holiday is meant to be a time to stop and tarry with God while also retaining the spiritual message of Sukkot.
Shemini Atzeret is celebrated in the global diaspora by following many of the observances of Sukkot, such as eating and/or sleeping in sukkot. Other common observances are the singing of various prayers, including the Tefillat Geshem (Rain Prayer).