Midras Imamat 23:34: Midrash Tehillim & Sifrei Devarim

"And in my days." During the holidays that You have given me, as it is said, "On the Sabbath day, on the Sabbath day, they shall stand before the Lord" (Leviticus 23:3). And it is written, "On the Day of Atonement" (Numbers 29:7) and it is written, "On the fourteenth day of the month of Tishrei, the holiday of Sukkot" (Leviticus 23:34). This refers to the days of my life.

(Devarim 16:13) "The festival of Succoth shall you make for yourself, seven days": Why is this stated? Because it is written (Vayikra 23:34) "the festival of Succoth, seven days, for the Most High," I might think (that it is being made for the L-rd); it is, therefore, written "shall you make for yourself."

(Devarim 16:15) "Seven days shall you celebrate before the L-rd your G-d": I might think that he sacrifices the festive offering (chagigah) all seven days; it is, therefore, written (Vayikra 43:41) "And you shall celebrate it" (the first day), and not all seven days.