On Rosh Hashana, as on other Hagim, one must insert "Ya’aleh V’Yavo" in the Birkat HaMazon. The question arises whether one who forgot to say Ya’aleh V’Yavo on Yom Tob must go back and recite Birkat HaMazon again, adding the Ya’aleh V’Yavo.
There is a major disagreement between the Rishonim (Early Authorities) on this subject. The Rambam, as well as the Ran, Rosh and Ritva, hold that one must repeat the Birkat HaMazon. On the other hand, Tosafot and the Rashba maintain that since there is no formal obligation to eat bread on Rosh Hashana and most other Hagim, forgetting the Ya’aleh V’Yavo does not invalidate the Birkat HaMazon. Only on the first night of Pesah and Succot, in which there is a Torah obligation to eat bread (or Massa), does the Ya’aleh V’Yavo become an integral part of Birkat HaMazon, and one who forget to insert it must repeat Birkat HaMazon.
The Shulhan Aruch (Siman 188) rules in accordance with the Rambam that one must repeat the Birkat HaMazon. However, the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) in Parshat Hukat (21) and Hacham Bension in his Or L’Sion (3:18:19) conclude that since there are authorities who disagree, this constitutes a Safek Berachot (a Halachic doubt whether to recite a Beracha) and one must be lenient and refrain from repeating the Birkat HaMazon.
SUMMARY
If one forgot to insert Ya’aleh V’Yavo in the Birkat HaMazon of Rosh Hashana and other Hagim, he should not repeat Birkat HaMazon, except for the first night of Pesah and Succot.
