This reading is from the special Torah reading for the first two days of Sukkot. The complete Torah reading for these days is Leviticus 22:26-23:44 and Numbers 29:12-16.
The Lord spoke to Moses, saying: Say to the Israelite people:
On the fifteenth day of this seventh month there shall be the Feast of Booths to the Lord, [to last] seven days. The first day shall be a sacred occasion: you shall not work at your occupations; seven days you shall bring offerings by fire to the Lord. On the eighth day you shall observe a sacred occasion and bring an offering by fire to the Lord; it is a solemn gathering: you shall not work at your occupations.
Those are the set times of the Lord that you shall celebrate as sacred occasions, bringing offerings by fire to the Lord--burnt offerings, meal offerings, sacrifices, and libations, on each day what is proper to it--apart from the sabbaths of the Lord, and apart from your gifts and from all your votive offerings and from all your freewill offerings that you give to the Lord.
Mark, on the fifteenth day of the seventh month, when you have gathered in the yield of your land, you shall observe the festival of the Lord [to last] seven days: a complete rest on the first day, and a complete rest on the eighth day. On the first day you shall take the product of hadar trees, branches of palm trees, boughs of leafy trees, and willows of the brook, and you shall rejoice before the Lord your God seven days. You shall observe it as a festival of the Lord for seven days in the year; you shall observe it in the seventh month as a law for all time, throughout the ages. You shall live in booths seven days; all citizens in Israel shall live in booths, in order that future generations may know that I made the Israelite people live in booths when I brought them out of the land of Egypt, I the Lord your God.
So Moses declared to the Israelites the set times of the Lord.