*Question:*

Hopefully it won’t rain on the first night of Sukkos, but if it does, what is the procedure to follow?

*Answer:*

If it rains on the first night of Sukkos [as well as the second night in Chutz L’aretz, L’chatchilah], before going and eating indoors it is proper to wait a little while in the hopes that the rain will subside and the Mitzvah of eating in the Sukkah will be able to be fulfilled.

Some say to wait half an hour, others say an hour or two, while some even say the obligation is to wait until Chatzos, halachic midnight.
If it does not stop raining the Sukkah should be entered anyhow, Kiddush should be recited in the Sukkah including the Bracha of Shehechiyanu, the hands should be washed and a kzayis of bread should be eaten, without reciting the Bracha of “Leishev BaSukkah”, and the rest of the meal should be finished indoors.

If the rain stops during the meal, a brief return to the Sukkah should be made, a “Leishev B’sukkah” recited and a little more than a K’Beitzah of bread eaten.

Afterwards, assuming the Sukkah is all wet and water is dripping from the Schach, the meal may be finished indoors.

Birchas Hamazon is recited indoors.

If the rain stops after the meal was finished and Birchas hamazon was already recited, it is still proper to go out at some point during the night to recite a Leishev B’sukah on a little more than a K’beitaza of bread.

If one already went to sleep, they do not need to wake up to do this.
(See Shulchan Aruch Siman 639 and Mishna Berura S”K 35 and 36)

From www.HalachaForToday.com