1)    One who makes the blessing of the grace after meals on Rosh Chodesh, says “Ya’aleh Veyavo”. If he forgot to do this, he does not go back, as there is no obligation to eat bread on Rosh Chodesh. (Shulchan Aruch 422)

2)    If a person commenced his meal on Rosh Chodesh, even if he finished after the end of the day of Rosh Chodesh, he nevertheless mentions Ya’aleh Veyavo when saying the grace after meals, as it depends on the beginning of the meal. (ibid)