Building the Sukkah:
 Sefer Haminhagim; Aruch Hashulchan 624/7; Kaf Hachayim 624/35
 So rules Rama in end of 624. Vetzaruch Iyun as to why this was omitted by Admur.
Other Opinions: Some Poskim rule one is to build the Sukkah before Yom Kippur in order so its Mitzvah becomes included in one’s verdict for the coming year. Others arbitrate that the walls are to be built before Yom Kippur while the Schach is placed after Yom Kippur. [Shaareiy Teshuvah]
 The reason for this is because one is not to delay the performance of a Mitzvah which he has the ability to fulfill. [ibid]