Charles Ellicott Commentary Leviticus 23:39

"Howbeit on the fifteenth day of the seventh month, when ye have gathered in the fruits of the land, ye shall keep the feast of Jehovah seven days: on the first day shall be a solemn rest, and on the eighth day shall be a solemn rest." — Leviticus 23:39 (ASV)

Also in the fifteenth day. —After the list of festivals discussed in this chapter has been summed up in Leviticus 23:37-38, the next five verses recur to the Feast of Tabernacles. The regulations are supplementary to those given before and embody a separate enactment.

When you have gathered in the fruit of the land. —That is, those products which ripen in the autumnal season, such as wheat, barley, oil, wine, etc.

You shall keep a feast to the Lord. —The Israelites are then to keep a festival in which they are to acknowledge the bounties of the Lord and express their gratitude to the Giver of all good things. For this reason this festival is also called “the Feast of Ingathering” (Exodus 23:16; Exodus 34:22).

On the first day shall be a sabbath. —Both on the first and last days of this festival there is to be abstention from all servile work. (See Leviticus 23:35-36.)