On the first two nights of Sukkot, eating in the sukkah is a Torah requirement that must be fulfilled. A person may not say, “I would rather skip dinner altogether and not eat in the sukkah.” One must, unless one is sick, have dinner in the sukkah.
On the other days of Sukkot there is no Torah requirement to eat in the sukkah, and one may decide (except on Shabbat and Yom Tov) to skip all meals and eat fruit or snacks outside the sukkah. Regular meals, however, must be eaten in the sukkah.
If it rains on the first two nights, one should wait a while – some say up to two hours – until it stops and then enter the sukkah, unless it is clear it will not stop. If one cannot wait, or if the rain will not stop, one enters the sukkah, recites Kiddush and Shehecheyanu, does not recite Leshev Basukkah, washes ones hands, eats some bread and returns indoors to eat. If it stops raining before one retires to bed, one returns to the sukkah, recites Leshev Basukkah, eats some bread and recites Birchat Hamazon.
Some see rain on Sukkot as Divine rejection. Others see it as Divine acceptance. After all, on Sukkot we pray for rain.