Shemini Atzeret (“The Eighth Day of Assembly”) celebrated on the 22nd day of the Hebrew month of Tishri, right after the Festival of Sukkot. According to the Torah (BeMidbar) – “On the eighth day you should hold a solemn gathering; you shall not work at your occupation”.
Rabbinic tradition teaches that on Shemini Atzeret the world is judged for water, or rainfall, in the upcoming year. In an agricultural society it was a very important day. It is on this day that we begin to ask for rain – Teffilat Geshrem. The prayer for rain begins on Shemini Atzeret and goes until Passover.
It is traditional to include Yizkor (a memorial service) during this day.