One who forgot to recite Minchah on Erev Shabbat must pray the Ma’ariv Shabbat Amidah twice, the first for Ma’ariv and the second for the Minchah that he missed. Even though that Minchah was supposed to be a weekday Amidah comprised of eighteen berachot, nevertheless, since the time to make it up falls on Shabbat, the wording of the Shabbat prayer is recited (Shulchan Aruch 108:9).
One who forgot to recite Ya’aleh V’Yavo in Minchah of Rosh Chodesh did not fulfill his obligation. If the following day is a second day of Rosh Chodesh, clearly he must pray a supplementary prayer after Ma’ariv for the missed Minchah and in both recite Ya’aleh V’Yavo. However, if the evening already begins a regular day, there is doubt concerning this issue. On the one hand, if the whole problem is that he did not mention Ya’aleh V’Yavo in Minchah, how would it help to pray another Amidah after Ma’ariv without Ya’aleh V’Yavo? On the other hand, he did not fulfill his obligation for Minchah, since at that time it was Rosh Chodesh and he failed to recite Ya’aleh V’Yavo, whereas if he were to pray a supplementary prayer he would fulfill his obligation because he would recite the appropriate prayers for that specific time. In practice, the ruling is that one is to recite Ma’ariv twice and make a stipulation that if he is not obligated to make up the missed prayer, the additional prayer he is praying should be considered voluntary. There is no need to introduce any new personal requests at this time.
The ruling is similar to the law regarding one who errs on Shabbat and instead of reciting Minchah of Shabbat recites the weekday wording without mentioning the sanctity of Shabbat. The law in that case requires that he recite two weekday Amidahs on Motza’ei Shabbat with a stipulation that if he is not obligated to make up the prayer, the additional prayer he is praying should be considered voluntary (Shulchan Aruch 108:11).
