The Feast of Tabernacles (Booths, Ingathering) - (Deuteronomy 16:13-15).
This feast is passed over very briefly, not because it was not important, for it was the feast at which the whole Law had to be read out every seven years (Deuteronomy 31:10-13), but because what Moses has been emphasising has already mainly been spelled out. This is very understandable given the context, but would be unlikely in someone who was inventing the speech afterwards. It is typical of a speaker who is conscious of the time his speech is taking and does not wish to weary his listeners by going through the same thing again and again.
Analysis in the words of Moses:
a You shall keep the feast of tabernacles seven days, after that you have gathered in from your threshing-floor and from your winepress (Deuteronomy 16:13).
b And you shall rejoice in your feast, you, and your son, and your daughter, and your man-servant, and your maid-servant, and the Levite, and the resident alien, and the fatherless, and the widow, that are within your gates (Deuteronomy 16:14).
b Seven days shall you keep a feast to Yahweh your God in the place which Yahweh shall choose (Deuteronomy 16:15 a).
a Because Yahweh your God will bless you in all your increase, and in all the work of your hands, and you shall be altogether joyful (Deuteronomy 16:15 b)
Note that in ‘a' they are to keep the feast in view of all the abundance of harvests that they have received, and in the parallel it is because Yahweh has blessed them in all their increase, and in all the work of their hands. Thus are they to be altogether joyful. In ‘b' they are all to rejoice in their feast from the highest to the lowest, none are to be excluded, and in the parallel they shall keep the feast to Yahweh their God for seven days in the place which He will choose.
‘ You shall keep the feast of tabernacles seven days, after that you have gathered in from your threshing-floor and from your winepress, and you shall rejoice in your feast, you, and your son, and your daughter, and your man-servant, and your maid-servant, and the Levite, and the resident alien, and the fatherless, and the widow, that are within your gates.'
The Feast of Tabernacles was celebrated at the end of the agricultural year. By this time not only had the barley and wheat harvest been gathered, but also the grape harvest and the summer fruits. The threshing floor and the winepress had done their job and it was now time to celebrate and look forward to the coming rains which would enable the commencing of the round all over again.
It was thus a special time of rejoicing, and all were to have a part in it. The description given, as constantly used in this regard in Deuteronomy, is intended to include everyone in the land who owes allegiance to Yahweh.
‘ Seven days shall you keep a feast to Yahweh your God in the place which Yahweh shall choose, because Yahweh your God will bless you in all your increase, and in all the work of your hands, and you shall be altogether joyful.'
Again the feast was to be kept for ‘seven days' demonstrating the divine perfection of the feast, and was to be held in the place where Yahweh had been pleased to take up His dwelling. This feast at the end of the agricultural ‘year' or season was to be held because Yahweh would have blessed their increase throughout the year, all their harvests would have been gathered in, and everything would have been more than satisfactory. Thus they would be altogether joyful, and they were to demonstrate the fact.
For details of the priestly functions at this feast see Numbers 29:12-38; Leviticus 23:33-36. There would, of course, also be a multitude of freewill offerings.