Albert Barnes 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 – Surely. The mode in which the Feast of Tabernacles is here reintroduced, after its mention in Leviticus 23:34-36, may suggest that this passage originally formed a distinct document.

The fruit of the land – that is, the produce, including the grain, the olives, the vintage and the fruits of all kinds. The time of year thus indicated would correspond in the Holy Land to the beginning of October. See Exodus 23:16 note.