Succoth is the joyous festival of booths, to remind us of our travels in the wilderness after the Exodus. We live in booths for seven days: it is a time to rejoice before our God. Shminiy Atzereth is the eighth day after the seventh day of Succoth. Its name means 'Eighth Ceasing,' or the day when we are restrained or hindered from our daily routine the day after the week of Succoth. 

Leviticus 33-43

YHVH spoke to Moses saying, 'Speak to the Children of Israel, saying: 'On the fifteenth day of this seventh month is the Festival of Succoth, a seven-day period for YHVH. On the first day is a holy convocation, you shall not do any laborious work. For a seven day period you shall offer a fire offering to YHVH; on the eighth day there shall be a holy convocation for you; and you shall offer a fire-offering to YHVH, it is an assembly, you shall not do any laborious work....

...On the fifteenth day of the seventh month, when you gather in the crop of the land, you shall celebrate YHVH's festival for a seven day period; the first day is a rest day and the eighth day is a rest day. You shall take for yourselves on the first day the fruit of a citron tree, the branches of date palms, twigs of a plaited tree (myrtle), and brook willows. And you shall rejoice before YHVH, your God, for a seven day period. You shall celebrate it as a festival for YHVH, a seven day period in the year, an eternal decree for your generations; in the seventh month shall you celebrate it. You shall dwell in booths for a seven day period; every native in Israel shall dwell in booths. So that your generations will know that I caused the Children of Israel to dwell in booths when I took them from the land of Egypt; I am YHVH, your God.