- One who is reciting Bircas Hamazonon Rosh Chodesh, whether by day or by night, and forgot to insert יַעֲלֶה וְיָבֹא, “may there rise and come” — if he already began the fourth blessing (“Who is good and Who does good”), he does not repeat Bircas Hamazon. But if he remembered right before beginning the fourth blessing, he should recite at that point the following formula: “Blessed is He Who gave New Moons to His people Israel as a remembrance.” This is recited without the Names of Hashem and “King of the Universe” (Shulchan Aruch and Kaf HaChaim 188); others maintain that it is said with the Names and Sovereignty (Mishnah Berurah ad loc.).
2. One who began his meal on Shabbos but finished it after the conclusion of Shabbos, or one who began a meal on Rosh Chodesh but finished it at night, after the conclusion of Rosh Chodesh, nevertheless inserts רְצֵה, “May it please You” or יַעֲלֶה וְיָבֹא, “may there rise and come,” since in this case we go by the beginning of the meal. (ibid.).