Albert Barnes Commentary Numbers 29:12-34

"And on the fifteenth day of the seventh month ye shall have a holy convocation; ye shall do no servile work, and ye shall keep a feast unto Jehovah seven days: and ye shall offer a burnt-offering, an offering made by fire, of a sweet savor unto Jehovah; thirteen young bullocks, two rams, fourteen he-lambs a year old; they shall be without blemish; and their meal-offering, fine flour mingled with oil, three tenth parts for every bullock of the thirteen bullocks, two tenth parts for each ram of the two rams, and a tenth part for every lamb of the fourteen lambs; and one he-goat for a sin-offering, besides the continual burnt-offering, the meal-offering thereof, and the drink-offering thereof. And on the second day [ye shall offer] twelve young bullocks, two rams, fourteen he-lambs a year old without blemish; and their meal-offering and their drink-offerings for the bullocks, for the rams, and for the lambs, according to their number, after the ordinance; and one he-goat for a sin-offering; besides the continual burnt-offering, and the meal-offering thereof, and their drink-offerings. And on the third day eleven bullocks, two rams, fourteen he-lambs a year old without blemish; and their meal-offering and their drink-offerings for the bullocks, for the rams, and for the lambs, according to their number, after the ordinance; and one he-goat for a sin-offering; besides the continual burnt-offering, and the meal-offering thereof, and the drink-offering thereof. And on the fourth day ten bullocks, two rams, fourteen he-lambs a year old without blemish; their meal-offering and their drink-offerings for the bullocks, for the rams, and for the lambs, according to their number, after the ordinance; and one he-goat for a sin-offering; besides the continual burnt-offering, the meal-offering thereof, and the drink-offering thereof. And on the fifth day nine bullocks, two rams, fourteen he-lambs a year old without blemish; and their meal-offering and their drink-offerings for the bullocks, for the rams, and for the lambs, according to their number, after the ordinance; and one he-goat for a sin-offering, besides the continual burnt-offering, and the meal-offering thereof, and the drink-offering thereof. And on the sixth day eight bullocks, two rams, fourteen he-lambs a year old without blemish; and their meal-offering and their drink-offerings for the bullocks, for the rams, and for the lambs, according to their number, after the ordinance; and one he-goat for a sin-offering; besides the continual burnt-offering, the meal-offering thereof, and the drink-offerings thereof. And on the seventh day seven bullocks, two rams, fourteen he-lambs a year old without blemish; and their meal-offering and their drink-offerings for the bullocks, for the rams, and for the lambs, according to their number, after the ordinance; and one he-goat for a sin-offering; besides the continual burnt-offering, the meal-offering thereof, and the drink-offering thereof." — Numbers 29:12-34 (ASV)

Feast of Tabernacles: (Compare to Leviticus 23:33 and following). The offerings required at this feast were the largest of all. It was especially one of thankfulness to God for the gift of the fruits of the earth; and the quantity and the nature of the offerings were determined accordingly.

(Numbers 29:32)

Stress is laid on the number seven, the holy symbolic covenant number, intimating that the mercies of the harvest accrued by virtue of God’s covenant. The diminishing number of bullocks sacrificed on the preceding days of the Feast (Compare to Numbers 29:13, Numbers 29:17, and so on), is adjusted simply to achieve this correspondence on the seventh day; but some have thought that the gradual evanescence of the Law until the time of its absorption in the Gospel is here presignified in the Law itself.