There is an important difference between the first night of Sukkot and the rest of the festival. During most of the festival, if one wants to eat a se’udat keva, he must eat it in the sukka, but one who wants to eat an akhilat ara’i may do so outside of the sukka. However, on the first night of Sukkot, there is an obligation to eat bread in the sukka. This obligation is based on a gezera shava from the identical language used by the Torah to describe the first night of Sukkot and Pesaḥ, from which the Sages derive that just as there is an obligation to eat matza on the first night of Pesaḥ, so too there is an obligation to eat bread in the sukka on the first night of Sukkot (Sukka 27a; SA 639:3). This also teaches us the importance of the first night of Sukkot, which lays the foundation for the entire festival.
To ensure that people fulfill the mitzva with appetite, one must avoid filling foods for the three hours before shki’at ha-ḥama on the eve of the festival (MB 639:27).
The obligation can be fulfilled from tzeit ha-kokhavim onward. Since this mitzva is derived from the mitzva of eating matza on the first night of Pesaḥ, one should eat the bread before midnight. Be-di’avad, one who did not manage to eat by midnight may do so until dawn (MB ad loc. 25-26; Peninei Halakha: Pesaḥ 16:31).
Before beginning to eat bread on the first night, one should have intent that this eating is to fulfill God’s commandment to us to eat in the sukka as a commemoration of the Exodus and of the clouds of glory with which He sheltered us in the wilderness. Le-khatḥila one should have this intention throughout the festival, but, be-di’avad, even on the first night one discharges his obligation as long as he knows he fulfills a mitzva by eating in the sukka (MB 625:1; see above, 1:4-5).
Just as we make sure to eat a kezayit of matza according to all opinions on the first night of Pesaḥ, so too we make certain to eat a kezayit of bread according to all opinions on the first night of Sukkot. Thus, one should eat at least half an egg’s bulk (keveitza) of bread, and some are stringent and eat more than a keveitza; this is an admirable practice. It is not necessary to squash the challah when calculating the volume of an egg; one may estimate based on the normal state of the challah. One should eat the necessary quantity of bread unhurriedly but steadily. If one stopped eating in the middle for longer than shi’ur akhilat pras (6-7 minutes), he must start again.
If it rains on the first night, some say there is no mitzva to eat a kezayit of bread in the sukka, as a mitzta’er (one who experiences discomfort) is exempt from the mitzva of sukka (Rashba; Smag). Others say that on the first night, even a mitzta’er must eat a kezayit of bread in the sukka (Rosh; Ran). In practice, it is proper to wait an hour or two in hopes that the rain will let up and it will be possible to fulfill the mitzva according to all. If the rain continues, or if the rain stopped but the water dripping from the waterlogged sekhakh makes it unpleasant to sit in the sukka, one should recite kiddush and She-heḥeyanu (on the festival itself) in the sukka and eat a kezayit of bread to fulfill the mitzva according to those who maintain that a mitzta’er is obligated to eat in the sukka on the first night. However, one should not recite the berakha of Leishev, since some maintain that even on the first night, a mitzta’er has no mitzva to eat in the sukka (Rema 629:5; MB ad loc. 35). If the rain stopped before midnight, and one would still get some enjoyment from eating bread, he should go to the sukka, recite “ha-motzi” and Leishev, and eat bread, thus fulfilling the mitzva according to those who maintain that the mitzva is to eat without discomfort.
Nevertheless, there are three reasons to be stringent and eat a bit more than a keveitza on the first night: 1) According to Ran, it is a mitzva to eat more than a keveitza of bread, because this is the quantity that obligates one to eat in the sukka. MB 639:22 states that le-khatḥila it is proper to follow this opinion. 2) According to Noda Bi-Yehuda, eggs nowadays are only half the size of the eggs in the time of the Sages, so the equivalent of eating half of a keveitza is eating the volume of an entire modern egg. Ashkenazim generally follow this position when it comes to Torah obligations. 3) Even though Sephardim do not generally give weight to the opinion of Noda Bi-Yehuda, they end up requiring a similar amount, because most Sephardim base their measurements on weight rather than volume, and it emerges that this quantity is more than an egg’s volume of bread. According to some, the reason for calculating based on weight rather than volume is that to calculate the volume of the bread properly, one would have to squash it (Peninei Halakha: Berakhot 10:5-6, and Peninei Halakha: Pesaḥ 16:23-24.) We see that le-khatḥila, there are three good reasons to eat a little more than a keveitza of bread or challah, approximately the size of a normal slice. Anyone who eats calmly but steadily will certainly be able to finish eating this amount of bread within the required amount of time (Peninei Halakha: Berakhot 10:7).