Torah law requires eating a Ke'zayit of bread in the Sukka on the first night of Sukkot. The Sages inferred this obligation from the association between Sukkot and Pesah which the Torah establishes by requiring that they both be observed on the fifteenth of their respective months (Nissan and Tishre). Just as on the first night of Pesah one bears a Torah obligation to partake of Masa, so does the Torah require partaking of bread in the Sukka on the first night of Sukkot.
As Hacham Ovadia Yosef notes in his Hazon Ovadia – Sukkot (p. 100; listen to audio for precise citation), the obligatory nature of the meal on the first night of Sukkot affects the Halachot governing a case of one who forgot to insert "Ya'aleh Ve'yavo" in Birkat Ha'mazon. According to Halacha, one who forgot to insert "Ya'aleh Ve'yavo" must repeat "Birkat Ha'mazon" if the meal he had eaten was required by virtue of the special occasion. Hence, if a person forgot to add "Ya'aleh Ve'yavo" in Birkat Ha'mazon on the first night of Sukkot, he must repeat Birkat Ha'mazon.
This applies only if the individual did not realize his mistake until after he began the Beracha of "La'ad Ha'Kel Avinu." If he caught his mistake earlier, after reciting the words "Baruch Ata Hashem" and before reciting "Boneh Yerushalayim," then he should immediately recite the words "Lamedeni Hukecha" and then return to "Ya'aleh Ve'yavo." If he realized his mistake only after he concluded the Beracha of "Boneh Yerushalayim" but before he began the Beracha of "La'ad Ha'Kel Avinu," then he should recite the special Beracha of "Baruch Ata…Asher Natan Shabbatot Ve'yamim Tovim…" which is printed in many Siddurim.
If one recited Birkat Ha'mazon on the first night of Sukkot and cannot remember whether or not he added "Ya'aleh Ve'yavo," must he repeat Birkat Ha'mazon?
Hacham Ovadia rules (ibid. p. 107; Yabia Omer, vol. 7, Orah Haim 28) that in such a case one may assume that he did, in fact, add "Ya'aleh Ve'yavo." The Hacham explains that the "Humrat Ha'yom," the unique stature and importance of the festival, likely caused the individual to remain aware of the Yom Tov and remember to make the required insertion in Birkat Ha'mazon.
There are, however, two important exceptions to this general rule concerning one who forgot to add "Ya'aleh Ve'yavo" in Birkat Ha'mazon on the first night of Sukkot. Firstly, if rain fell on the first night, and a person therefore ate the meal in his home, he does not repeat Birkat Ha'mazon if he forgot to add "Ya'aleh Ve'yavo." Since the Torah requires eating a meal in the Sukka on this night, and not in one's home, an individual in such a case has not eaten an obligatory meal, and it is therefore not necessary to repeat Birkat Ha'mazon.
Secondly, the requirement to repeat Birkat Ha'mazon on the first night of Sukkot does not apply to women, who are exempt from the obligation of Sukka. Given their exemption, their meal on the first night of Sukkot does not constitute an obligatory meal, and they therefore do not repeat Birkat Ha'mazon if they forgot to add "Ya'aleh Ve'yavo."
These Halachot apply only on the first night of Sukkot (and the first night of Pesah); if one forgot to insert "Ya'aleh Ve'yavo" in Birkat Ha'mazon during the daytime meal on the first day of Sukkot, or on another night or day of Sukkot, he does not repeat Birkat Ha'mazon. Since there is no Torah obligation to eat a bread meal during the day of Yom Tov, one who forgot to add "Ya'aleh Ve'yavo" is not required to repeat Birkat Ha'mazon. Nevertheless, if he caught his mistake before he began the Beracha of "La'ad Ha'Kel Avinu," then he should insert at that point the Beracha of "Asher Natan Shabbatot Ve'yamim Tovim…" as mentioned above.
Summary: One who forgot to recite "Ya'aleh Ve'yavo" in Birkat Ha'mazon on the first night of Sukkot, and did not realize his mistake until after he began the Beracha of "La'ad Ha'Kel Avinu," must repeat Birkat Ha'mazon. If he is unsure as to whether he added "Ya'aleh Ve'yavo," he may assume that he did. If a person ate indoors on the first night of Sukkot due to rain and the like, and he forgot to add "Ya'aleh Ve'yavo," he does not repeat Birkat Ha'mazon. A woman who forgot to add "Ya'aleh Ve'yavo" on Sukkot in any event does not repeat Birkat Ha'mazon. Throughout the rest of Sukkot, both men and women do not repeat Birkat Ha'mazon if they forgot to add "Ya'aleh Ve'yavo."
