135:3 Eating in the succah on the first night of Succos is obligatory. One must eat at least a k’zayis (an olive-sized portion – about an ounce) of bread in the succah, even if eating in the succah makes him uncomfortable. (One should optimally eat more than a k’beitza – an egg-sized portion, more like two or three ounces – in order to fulfill the mitzvah according to all authorities’ opinions – Mishnah Brurah 639:22.) If rain is falling and one believes that it will stop after an hour or two, he should wait to make kiddush and eat in the succah. If he sees that the rain will not stop, or if he waited and it did not stop, he makes kiddush in the succah – including the bracha of shehechiyanu, intending it to include the mitzvah of succah. However, one does not recite leishev basuccah (that G-d commanded us to dwell in the succah). He washes his hands and recites HaMotzi, then he eats a k’zayis of bread without interruption and goes in the house to finish his meal. One should intend while washing his hands and reciting HaMotzi that he intends also to eat in the house. If the rain stopped before bentching, he should return to the succah (even after midnight – Shaar HaTziyon 639:68), recite leishev basuccah, and eat a little more than a k’beitza of bread. He then bentches. If the rain stopped after he already bentched, he goes out to the succah, washes his hands a second time, eats a little more than a k’beitza of bread with the bracha “leishev basuccah,” and then bentches again. (If he has already gone to bed when the rain stops, he need not get up in order to return to the succah – MB 639:36.) If water is still dripping from the schach and there is a succah in his neighborhood that has a retractable roof that was closed and re-opened after the rain, he should go there in order to be able to eat his bread with a joyful heart.
135:4 There is also an obligation to eat in the succah on the second night of Succos (outside of Israel), even if doing so makes one uncomfortable. The laws are the same as on the first night, which have already been described. There is one difference: if one saw that the rain was not stopping, or if he waited and it did not stop, before bentching he goes into the succah and eats at least a k’zayis of bread without reciting the bracha of leishev basuccah. He then returns to his house and bentches.