God established the Feast of Booths (aka, Tabernacles, Sukkot, In-gathering) as a SEVEN day holiday that lasts EIGHT days. Sukkot (pron sue-COAT) refers to the temporary dwellings (aka, tabernacles, booths) that the Jews were commanded to live in in memory of the period of wandering.
Leviticus 23 says,
33 The LORD said to Moses: 34 Tell the Israelites: The fifteenth day of this seventh month is the LORD’s feast of Booths, which shall continue for seven days. 35 On the first day, a declared holy day, you shall do no heavy work.
36 For seven days you shall offer an oblation to the LORD, and on the eighth day you will have a declared holy day. You shall offer an oblation to the LORD. It is the festival closing. You shall do no heavy work.
37 These, therefore, are the festivals of the LORD which you shall declare holy days, in order to offer as an oblation to the LORD burnt offerings and grain offerings, sacrifices and libations, as prescribed for each day, 38 in addition to the LORD’s sabbaths, your donations, your various votive offerings, and the voluntary offerings that you present to the LORD.
39 On the fifteenth day, then, of the seventh month, when you have gathered in the produce of the land, you shall celebrate the feast of the LORD for a whole week. The first and the eighth day shall be days of rest.
40 On the first day you shall gather fruit of majestic trees, branches of palms, and boughs of leafy trees and valley willows. Then for a week you shall make merry before the LORD, your God. 41 You shall keep this feast of the LORD for one whole week in the year. By perpetual statute throughout your generations in the seventh month of the year, you shall keep it.
42 You shall dwell in booths for seven days; every native-born Israelite shall dwell in booths, 43 that your descendants may realize that, when I led the Israelites out of the land of Egypt, I made them dwell in booths. I, the LORD, am your God.
I think the Eighth Day is a fore-shadowing of the last 2000 years when God gathered in the Gentiles and of the time after Jesus whips Satan’s butt at Armageddon when God will gather in all the people He ever made for Final Judgment.