Siman 625
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א בַּסֻכּוֹת תֵּשְׁבוּ שִׁבְעַת יָמִים וְגו' כִּי בַּסֻכּוֹת הוֹשַׁבְתִּי אֶת בְּנֵי יִשְׂרָאֵל הֵם עַנְנֵי כָּבוֹד שֶׁהִקִּיפָם בָּהֶם לְבַל יַכֵּם שָׁרָב וָשֶׁמֶשׁ וּמִצְוָה לְתַקֵּן הַסֻּכָּה מִיָּד לְאַחַר יוֹם כִּפּוּר, דְּמִצְוָה הַבָּאָה לְיָדוֹ אַל יַחְמִיצֶנָּה (מַהֲרִי"ל).
1 On Succot we shall dwell for 7 days etc. Because on Succot I protected the children of Israel. These refer to the clouds of glory who protected them from all the intense heat and the sun of the desert. And it is a Mitzvah to begin building the Succah immediately after Yom Kippur. For if a Mitzvah that comes to one's hands, one should not delay performing it.[Maharil]