By Rabbi Doniel Neustadt
Discussed below:

1. How does one properly fulfill the mitzvah of eating in the succah on the first night of Succos?
2. Are we required to eat a k’zayis of bread in the succah when it is raining?

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Question: How does one properly fulfill the mitzvah of eating in the succah on the first night[1] of Succos?
Discussion: Every adult male is Biblically obligated to eat a k’zayis of bread in a succah on the first night of Succos. The Talmud[2] derives this obligation from the similar obligation of eating a k’zayis of matzah on the first night of Pesach. Since these two obligations are closely related, their halachos are similar in several respects. Like all mitzvos, this mitzvah, too, can only be properly fulfilled if there is prior planning and clear knowledge of all the requirements. Indeed, the poskim remind us that in the late afternoon of erev Succos, one should not eat so much food or wine that he will be unable to eat the k’zayis of bread with a good appetite.[3]
Let us review the pertinent halachos:
When is the k’zayis of bread eaten?
Preferably, one should sit down to eat the k’zayis of bread as soon as possible. Unnecessary delays should be avoided.[4]
One cannot eat the k’zayis of bread (nor make the Kiddush that precedes it[5]) until it is definitely night.[6] While there are several opinions concerning when “night” is, most communities in the United States consider “night” to be no earlier than fifty minutes after sundown.[7] Others are more stringent and wait until seventy-two minutes have passed.[8] One who mistakenly ate the k’zayis before it is definitely night must eat another k’zayis of bread in order to fulfill the mitzvah.[9] [If, mistakenly, one ate his k’zayis of bread even earlier than sundown, not only must he eat another k’zayis but he must also repeat the blessing of leishev basuccah.]
The k’zayis of bread may not be eaten after midnight.[10] Bedieved, though, one who did not eat before midnight should do so after midnight and recite the proper blessing over the succah.[11]
In fact, how much bread must be eaten?
There are various views among the poskim about the exact measurement of a k’zayis. Since this is a Biblical obligation, it is proper to be stringent and eat at least 1.7 fl. oz. of bread, although, bedieved, one who ate 1 fl. oz. of bread fulfilled his obligation.
There is a view among the Rishonim[12] that holds that the minimum amount of bread one is obligated to eat in the succah on the first night is slightly more than a k’beitzah, not merely a k’zayis. Although the basic halachah does not require the larger amount,[13] still it is proper to satisfy that view as well.[14] The amount to be eaten (to satisfy all views), therefore, is 3.5 fl. oz. of bread.[15] This is approximately a third of a challah roll, or almost an entire middle slice of a 2-lb. loaf of rye bread.
The bread (whether it is a k’zayis or slightly more than a k’beitzah[16]) should be eaten within a timespan of no more than two minutes.[17] If that is difficult, one may take up to three or four minutes to eat the proper amount.[18] One may not talk [or eat other foods] until he has chewed and swallowed the full amount.[19] Lechatchilah, it is proper to chew and then swallow a k’zayis of bread in its entirety.[20]
Before eating the bread, one must have in mind that he is about to fulfill the Biblical mitzvah of eating bread on the first night of Succos.[21] [In addition to this kavanah, one should bear in mind the reasons behind the mitzvah of succah. According to some poskim, failure to have such intent invalidates the mitzvah. Mishnah Berurah, however, rules that bedieved one fulfills his obligation even if he did not concentrate on the reasons for the mitzvah.] If one fails to have this intent and eats the bread as he normally does every Shabbos or Yom Tov, it is questionable if he has fulfilled the mitzvah.[23] In any case, he should eat another portion of bread with the proper intent.[24]
Who is obligated in this mitzvah?      
One is obligated to eat the minimum amount of bread even if he does not enjoy it and even if it causes him distress.[25] Even a person who is classified as a choleh sh’ein bo sakanah is obligated to eat a k’zayis of bread.[26]
One who cannot eat bread does not fulfill his obligation by eating cake, etc.[27] Only bread made of one of the five species of grain is valid. [Some poskim maintain that only “water” challah may be used to fulfill this mitzvah, not challah (or bread) to which oil, sugar or eggs were added. But most poskim are not particular about this stringency.][28]
Women are exempt from this mitzvah, but if they do eat the required amount of bread in the succah, it is considered a mitzvah and they may recite the blessing over the succah.[29]
Children over the age of chinuch are taught to fulfill this mitzvah as well.

Question: Are we required to eat a k’zayis of bread in the succah when it is raining?
Discussion: There are several disputes among the poskim concerning the obligation to eat in the succah on the first night of Succos if it is raining. The following points are debated:

• If rain is falling, is one obligated to eat in the succah or not?
• If it is raining, is one obligated to wait and see if the rain will stop so that he can eat in a dry succah?
• If one does eat in the succah while it is raining, can a blessing be recited?
• If a person ate in the succah while it was raining and then the rain stopped, is he required to eat in the succah again?
• If a person ate in the succah while it was raining and then went to sleep, is he obligated to get out of bed to eat again once the rain has stopped?

Since there are various opinions on all of these issues, the following is a summary of the majority view[30] and the accepted practice:
If it is raining steadily and there is a reliable weather forecast for rain all night, one should make Kiddush [with shehecheyanu] and eat a k’zayis [or slightly more than a k’beitzah] in the succah. No blessing over the succah is recited. The rest of the meal is eaten in the house.[32]
If (as is often the case) there is no reliable weather forecast and there is a possibility that the rain will stop, e.g., it is drizzling or it is raining off and on, it is proper to wait an hour or two for the rain to subside.[33] The poskim agree, however, that if the delay will disturb the dignity and pleasure of the Yom Tov, or if the family is hungry and/or tired, there is no obligation to wait.
If the rain stops while the meal is being eaten in the house or even after the meal is finished, one is obligated to go to the succah, recite leishev basuccah, and eat at least slightly more than a beitzah[34] of bread. Even if the rain stops after midnight, that amount of bread must be eaten in the succah. If one has already gone to bed and then the rain stops, there is no obligation to get out of bed in order to eat in the succah.[35]

Footnotes
[1]    Outside of Eretz Yisrael, the mitzvah is repeated on the second night as well.
[2]    Succos 27a.
[3]    Mishnah Berurah 639:27.
[4]    Mateh Efrayim 625:42, 44.
[5]    Beiur Halachah 639:3, s.v. v’lo.
[6]    Rema, O.C. 639:3.
[7]    Under extenuating circumstances, there are opinions that permit eating the bread a few minutes earlier. In Eretz Yisrael, the widespread custom is to consider 20-30 minutes after sundown as “night.”
[8]    Since this is a Biblical mitzvah, it is proper—weather permitting—to wait for 72 minutes after sundown to satisfy the more stringent views.
[9]    Mishnah Berurah 639:25.
[10] Rema, O.C. 639:3.
[11] Mishnah Berurah 639:26. In this case, the blessing is only recited if at least a k’beitzah is eaten.
[12] Quoted by the Ritva and Ran in Succos 27b.
[13] O.C. 639:3.
[14] Mateh Efrayim 625:51; Mishnah Berurah 639:22.
[15] The amount of a beitzah according to the Chazon Ish.
[16] Mateh Efrayim 625:52 and Elef Lemateh 87; Kaf Hachayim 629:51. Rav S.Z. Auerbach, however, maintains that for the amount of a k’beitzah, double the amount of time is allowed; see end notes to Succah Hashalem, #27.
[17] Mishnah Berurah 639:22. Children under bar mitzvah may take up to 9 minutes; Rav S.Z. Auerbach (Shemiras Shabbos Kehilchasah 54, note 130).
[18]   See Shiurin Shel Torah, pg. 67 and Igros Moshe, O.C. 4:41.
[19]   Kaf Hachayim 639:50.
[20]   Mateh Efrayim 625:52. Mishnah Berurah, though, does not mention this.
[21]   Mateh Efrayim 625:51; Mishnah Berurah 625:1.
[22] Bikurei Yaakov 625:3 based on Bach.
[23]   See Chida (Simchas Haregel, quoted in Moadim U’zemanim 6:69) who questions if one has fulfilled his obligation in this case. See, however, Mishnah Berurah 60:10, quoting the Chayei Adam.
[24]   Mateh Efrayim 625:53.
[25]   Beiur Halachah 639:3, s.v. b’leil.
[26]   Bikurei Yaakov 639:6,24; Aruch Hashulchan 639:17.
[27]   Mishnah Berurah 639:21.
[28]   See Yechaveh Da’as 4:37, Nishmas Avraham, O.C. pg. 320, Moadim U’zemanim 1:86 and Piskei Teshuvos 639:13 for the various views.
[29]   Sephardic women, though, should not recite the blessing for the succah or for any mitzvah which they are not obligated to perform, such as lulav, shofar, etc.
[30]   Based on rulings of Mateh Efrayim and Mishnah Berurah.
[31]   Mateh Efrayim 625:51, 62 and Elef Lemateh 84. See, however, Ketzei Hamateh, who maintains that in this case a k’zayis is sufficient according to all of the views quoted earlier.
[32] When reciting hamotzi, one should have in mind that he will recite Birkas hamazon in the house.
[33]   Some poskim are more stringent and recommend waiting until midnight.
[34]   In this case, because of the blessing which is to be recited, a k’zayis is not enough.
[35]   A minority opinion (Moadim U’zemanim 1:86, based on his understanding of the Gaon of Vilna; Rav M. Soloveitchik, quoted in Reshimos Shiurim, Succah, pg. 92, and in Mesorah, vol. 14, pg. 57) maintains that even after going to sleep, one is obligated to wake up and get out of bed in order to eat in the succah.