May one build his Sukkah on Erev Sukkos or on Erev Shabbos before Sukkos?[1]

Until midday: It is permitted even initially to build the Sukkah on Erev Shabbos, or Erev Sukkos, up until midday.

Past midday-before Mincha Ketana: Some Poskim[2] rule that one may only build his Sukkah up until midday [i.e. Mincha Gedola] of Erev Shabbos and Erev Yom Tov. [Thus, in years that there is a Shabbos between Yom Kippur and Sukkos, one is to stop building his Sukkah by midday, and he is annually to make sure that his Sukkah is fully built by midday of Erev Sukkos.] However, other Poskim[3] rule that he may be lenient to continue building the Sukkah until Mincha Ketana, which is 2.5 hours before sunset. Practically, one is to initially try to have his Sukkah built by midday, although one who chooses may be lenient to do so up until Mincha Ketana.[4] Certainly, if on Erev Sukkos the Sukkah has not yet been built, it may be built even past midday.

Past Mincha Ketana: It is forbidden to build the Sukkah past the time of Mincha Ketana on Erev Shabbos.[5] However, on Erev Sukkos, if it is already past 2.5 hours before sunset and one has still not built his Sukkah, it may still be built up until the start of Yom Tov.[6] It is permitted on Erev Shabbos to have the Sukkah built through a gentile even past the time of Mincha Ketana.[7]

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[1] See Piskeiy Teshuvos 625:4

[2] P”M 625; M”B 625:2; Mateh Efrayim 625:9 regarding Baal Nefesh; So rule regarding Melacha on Erev Shabbos: 1^st opinion in Michaber 251:1; 1^st opinion in Admur 251:2 regarding Melacha on Erev Shabbos and from his final ruling it is evident that there is room to be stringent; Mordechai and Tur, and so rules the Bach and Levush

[3] Mateh Efrayim 625:9; Bikureiy Yaakov 625:5; So rule regarding Melacha on Erev Shabbos: 2^nd opinion in Michaber 251:1; 2^nd opinion in Admur 251:2 and final ruling that one may be lenient by a dispute regarding a Rabbinical prohibition; Admur 256:1; Taz 251:1 one who is lenient does not lose; Magen Avraham 251:4; Rashi; Ketzos Hashulchan [69:6

[4] See conclusion of Admur 251:1 that one who is lenient does not lose the Siman Bracha

[5] Admur 251:2; Michaber 251:1

[6] See Mateh Efraim 625:28; Piskeiy Teshuvos ibid

The reason: As a) It is for the need of Yom Tov, and B) It is for the sake of a Mitzvah. Vetzaruch Iyun from the wording or Mateh Efrayim and M”B ibid, although perhaps they were merely referring to by what time should one plan to have it built by, and not bedieved if one did not yet do so.

[7] P”M 625 A”A 1