Leviticus 23:39

Also in the fifteenth day of the seventh month
The month Tisri or September, the same month, and the same day of the month before observed; only another end and use of this feast is remarked, which was to give thanks for the fruits of the earth gathered in, as follows:

when ye have gathered in the fruit of the land;
the barley, wheat, oil and wine, and all others, this being now autumn, when the several fruits were ripe and gathered: ye shall keep a feast unto the Lord seven days; not different from that before mentioned, but the same, one design of which is here suggested, to give thanks for the fruits of the earth: hence this feast is sometimes called the feast of ingathering, ( Exodus 23:16 ) ( 34:22 ) ; as another use of it is after mentioned, to commemorate the children of Israel dwelling in booths in the wilderness:

on the first day [shall be] a sabbath, and on the eighth day [shall
be] a sabbath;
because on both there was a cessation from servile work, ( Leviticus 23:35 Leviticus 23:36 ) .