Sukkah, Chapter Two, Mishnah 6. Here we have a further discussion about the obligation to eat in the sukkah.
The View of Rabbi Eliezer:
"Rabbi Eliezer omer: arba esreh se'udos chayav adam le'echol basukkah" - a person is obligated to eat fourteen meals in the sukkah: one by day and one by night, on each of the seven days of the festival.
The basis for this is the wording of the Torah: "Basukkos teishvu shivas yamim" - one is obligated to dwell in the sukkah for seven days, and from here Chazal derived the principle "teishvu ke'ein taduru" - dwelling in the sukkah must be like the way one dwells in a house. Just as at home a person eats a night meal and a day meal, so too he must eat a night meal and a day meal in the sukkah.
The View of the Sages:
The Sages disagree and say "ein lo kitzbah" - there is no fixed number of meals, since even at home a person is not obligated to eat, and at times he skips a meal. Therefore there is no obligation to eat any particular number of meals. But they added: "chutz mileilei yom tov harishon shel chag bilvad" - on the first night of Sukkos there is an obligation to eat in the sukkah.
This obligation applies even under difficult circumstances: ordinarily one does not eat in the sukkah when rain is falling, but on the first night of Sukkos one eats in the sukkah even when there is no other option.
The Source of the Obligation on the First Night:
The first night of Sukkos is derived from the first night of Pesach. Just as on the first night of Pesach a person is obligated to eat matzah - even if he is not hungry, and if he does come to eat a meal he is obligated in matzah - so too on the first night of Sukkos he is obligated to eat bread in the sukkah. The derivation is made from the fact that regarding both festivals the Torah uses the expression "the fifteenth," since both fall on the fifteenth of the month. This is a gezeirah shavah that bridges the two passages - one dealing with Sukkos and the other with Pesach: just as on Pesach there is an obligation to eat on the first night, so too on Sukkos. But on the remaining days there is no obligation, according to the Sages.
The Continuation of the Mishnah - the Law of Compensation:
Rabbi Eliezer went on to say: "Mi shelo achal leilei yom tov harishon yashlim beleilei yom tov ha'acharon" - one who did not eat on the first night of Sukkos can make it up on the night of the final festival day, which is Shemini Atzeres.
The Sages disagree and say "ein lo tashlumin" - there is no way to make it up. Regarding this they said that this is an example of what is stated in the verse in Koheles: "Me'uvas lo yuchal liskon" - something crooked that cannot be straightened, "vechesron lo yuchal lehimanos" - a deficiency that cannot be counted. Once the day has passed and the opportunity to eat in the sukkah was not used, there is no longer any way to make it up; the opportunity is lost and nothing can be done about it.
The Gemara's Question:
The Gemara finds a contradiction between Rabbi Eliezer's first statement in the Mishnah - that one must eat fourteen meals in the sukkah - and his final statement, that one who did not eat on the first night of Sukkos makes it up on the last night, Shemini Atzeres. The two statements do not fit together.
The Rishonim disagree as to what exactly the root of the contradiction is: is it rooted in the choice of wording alone - for he did not say that a person failed to eat one of the fourteen meals, but rather said "one who did not eat on the first night of the festival," as though the first night were the only mitzvah - and there are additional ways to explain the difficulty.
In the end the Gemara concludes that Rabbi Eliezer retracted his original position in the Mishnah, according to which one must eat fourteen meals, and ultimately held like the Sages - that the only time there is an obligation to eat in the sukkah is the first night of Sukkos.