The Festival of Sukkot contains an enigma: is it prescribed in the Torah to be 7 days long or 8? The text in Lev 23 seems a bit ambiguous: (vv. 33-36):
“Again Adonai spoke to Moses, saying, “Speak to the sons of Israel, saying, ‘On the fifteenth of this seventh month is the Feast of Booths for seven days to Adonai. ‘On the first day is a holy convocation;
you shall do no laborious work of any kind. ‘For seven days you shall present an offering by fire to Adonai. On the eighth day you shall have a holy convocation and present an offering by fire to Adonai; it is an assembly. You shall do no laborious work.”
The festival is specifically detailed as lasting seven days, yet a command is given regarding the eighth day. This is reiterated a few verses later in Lev 23: (v.39):
“‘On exactly the fifteenth day of the seventh month, when you have gathered in the crops of the land, you shall celebrate the feast of Adonai for seven days, with a rest on the first day and a rest on the eighth day.
In fact, this eighth day is referred to as “an assembly” or a “holy assembly,” which is עצֲרֶֶת in the Hebrew of Lev 23:36, and thus שְׁמִניִ עֲצֶרֶת , Shemini Atzeret means “eighth-day assembly.” Atzeret is from the root עצר (‘atzar) which means “to restrain, hold back,” and thus, the Sages understand the designation to be referring to a “holding on to the festival” so as to restrain it from ending—making the festival last for yet another day. But since the text clearly states that one is to dwell in the Sukkah for 7 days (not 8), the eighth day was understood by the Sages as a separate festival, yet one which is, in every way, connected to the 7 days of Sukkot, and maintains the character of Sukkot……