One must recite Ya’aleh Ve-yavo in Birkat Ha-mazon as well. Even though one is not obligated to eat a festive meal on Rosh Ĥodesh, one must mention Rosh Ĥodesh when reciting Birkat Ha-mazon because of the importance of the day, on which the musaf offering was brought (Shabbat 24a; Tosafot ad loc.). Ya’aleh Ve-yavo is inserted in the berakha of Raĥem, because both are prayers of supplication.

If one forgot to recite Ya’aleh Ve-yavo in Birkat Ha-mazon, he need not repeat the prayer. This is because one who forgets to mention the sanctity of a particular day must repeat Birkat Ha-mazon only on days when there is an obligation to eat a meal with bread, like Shabbat and Yom Tov. On Rosh Ĥodesh and Ĥol Ha-mo’ed, however, one is not obligated to eat such a meal. Therefore, from the perspective of the sanctity of the day, one need not recite Birkat Ha-mazon at all. Consequently, one need not repeat it if he accidentally omitted Ya’aleh Ve-yavo (sa 424:1).[13]

One who began a meal on Rosh Ĥodesh, and managed to eat a kezayit (olive’s bulk) of bread before sunset, must recite Ya’aleh Ve-yavo in Birkat Ha-mazon, even if he continued eating long after tzeit ha-kokhavim (also called “tzeit”; meaning, “the emergence of stars,” i.e., nightfall), because the meal began on Rosh Ĥodesh (sa 188:10; see Kaf Ha-ĥayim 188:43 for the opposing view).

If one began to eat on the eve of Rosh Ĥodesh, and finished his meal after tzeit, he must recite Ya’aleh Ve-yavo, provided that he ate a kezayit of bread after Rosh Ĥodesh began (sa 271:6, mb 271:29).[14]