We have already observed that the Feast of Tabernacles began on the fifteenth day of the seventh month, and that it lasted for seven days, and that they made burnt offerings everyday for seven days, but the Lord emphasized it again,
“Also on the fifteenth day of the seventh month, when you have gathered in the fruit of the land, you shall keep the feast of the LORD for seven days; on the first day there shall be a sabbath-rest, and on the eighth day a sabbath-rest. And you shall take for yourselves on the first day the fruit of beautiful trees, branches of palm trees, the boughs of leafy trees, and willows of the brook; and you shall rejoice before the LORD your God for seven days. You shall keep it as a feast to the LORD for seven days in the year. It shall be a statute forever in your generations. You shall celebrate it in the seventh month. You shall dwell in booths for seven days. All who are native Israelites shall dwell in booths” (Leviticus 23.39–42).
There was no way for the Israelites to miss what they were supposed to do, for how long they were supposed to do it, and when they were to begin.
When the Lord says something once, that is enough to establish the law, but when He repeats it, you know it is highly important to Him.