326. Forgetting Yaaleh v'Yavo

Tefillah u’Birkas Kohanim 10:10

Let’s say that someone makes a mistake and neglects to recite yaaleh v'yavo on a day that requires it. If he remembers before finishing Shemoneh Esrei, he should go back to the bracha of R’tzei and say it. If he remembers after he finishes davening, he must repeat Shemoneh Esrei from the beginning. If he recites the prayers after Shemoneh Esrei and he remembers after he has finished these prayers but before taking his three steps back, he goes back to R'tzei.

Tefillah u’Birkas Kohanim 10:11

This is the case on chol hamoed, or in shacharis or mincha on Rosh Chodesh. In maariv on Rosh Chodesh, however, one need not repeat Shemoneh Esrei if he forgets yaaleh v’yavo.