Leviticus 23:34-36: “On the fifteenth day of the seventh month is the Feast of Tabernacles … On the first day shall be a holy convocation; you shall do no manner of creative labor. For seven days you will bring a fire offering to God; on the eighth day shall be a holy convocation for you … it is a day of solemn assembly; you shall do no manner of creative labor.”

According to Leviticus 23, the Feast of Tabernacles (Sukkot, including Shemini Atzeret) lasts for eight days. Creative labor, however, is prohibited only on the first and the eighth days. Why then will Jews around the world (except in Israel) celebrate the first two days and the eighth and ninth days as festival days, refraining from creative labor?

The rabbinically ordained Yom Tov Shaynee Shel Galuyot (the second festival day of the diaspora) is the result of our people’s geographic spread. As Jews moved farther from the sphere of influence of Jerusalem (considered what would then have been a ten day journey), maintaining an accurate Jewish calendar became more difficult.

Before the Jewish calendar was fixed by mathematical calculation in 350 C.E. (approximately), the new month was determined by the Sanhedrin based on the testimony of witnesses who had seen the new moon. As the diaspora spread, it became impossible to inform all distant communities when the new month had been declared, so a precautionary second day was added for those distant locations. Far better to sanctify the extra day than to risk violating a day that was actually Yom Tov.

After the calendar was set, it was decided that the custom of Yom Tov Shaynee Shel Galuyot be honored by remaining in practice. To this day, Jews in Israel celebrate one day of Yom Tov, while Jews throughout the rest of the world celebrate two.