Today and tomorrow, Tuesday and Wednesday, will mark Rosh Chodesh Marcheshvan. Let us therefore discuss some laws pertaining to Rosh Chodesh.
Question: If one forgets to mention “Ya’aleh Ve’Yavo” during Rosh Chodesh prayers, must one repeat the Amidah prayer again?
Answer: It is well-known that during Rosh Chodesh prayers, the “Ya’aleh Ve’Yavo” paragraph, which discusses the idea of the new month coming upon us amid blessing and goodness, is added in the middle of the “Retzeh” blessing of the Amidah prayer. This text was established by the same Sages who established the rest of the prayer texts.
The Gemara (Berachot 30b) states that if one mistakenly forgets to insert “Ya’aleh Ve’Yavo” into the Amidah prayer of Rosh Chodesh, one must repeat the entire Amidah prayer, for if one prays without mentioning Rosh Chodesh, it is as if he has omitted any other part of the Amidah prayer which was entirely instituted by the Sages; one must therefore repeat the entire Amidah. (Anytime we mention praying again, this refers to repeating only the Amidah prayer.)
The Gemara (ibid.) explains that according to the opinion of Rabbi Yochanan, when we said that one who forgets to add “Ya’aleh Ve’Yavo” must repeat the Amidah prayer, this refers only to the regular members of the congregation; however, if the Chazzan forgets to insert “Ya’aleh Ve’Yavo” into his repetition of the Amidah prayer, he need not repeat the Amidah, for this will over-burden the congregation since they will have to wait until the Chazzan recites the repetition of the Amidah once again. Our Sages therefore did not require the Chazzan to repeat the Amidah and his mention of Rosh Chodesh in the Mussaf prayer is sufficient. However, if any other person besides for the Chazzan who forgets to add “Ya’aleh Ve’Yavo” in the Rosh Chodesh Amidah prayers, whether one is praying with a Minyan or alone in one’s home, one must repeat the Amidah prayer once again. It is preferable for one to don one’s Tefillin again before praying a second time (as Maran zt”l discusses at length in his Halichot Olam-beginning of Volume 5).
Summary: If one forgets to insert “Ya’aleh Ve’Yavo” in the Amidah prayer of Rosh Chodesh and remembers his error only after concluding the Amidah, one must recite the Amidah prayer over again. If one remembers before the conclusion of the Amidah prayer, one must return to the beginning of the “Retzeh” blessing, insert “Ya’aleh Ve’Yavo” as per the Halacha, and then continue with the rest of the Amidah as usual. (See Maran Ha’Shulchan Aruch, Chapter 126)
All this applies only to the Shacharit and Mincha prayers of Rosh Chodesh. However, if one forgets to mention “Ya’aleh Ve’Yavo” within the Amidah of the Arvit prayer of Rosh Chodesh, one need not repeat the Amidah prayer at all.