Halacha requires inserting Ya’ale Va’yabo in the Amida prayer at Arbit, Shaharit and Minha on Rosh Hodesh. The paragraph is inserted in the Beracha of "Reseh," just before the concluding passage, "Ve’Ata Berahamecha Harabim Tahpotz Banu…"
A person who forgot to recite Ya’ale Ve’yabo in the Amida during Arbit on Rosh Hodesh does not repeat the Amida. Arbit differs in this regard from Shaharit and Minha, when a person must repeat the Amida if he forgets to add Ya’ale Ve’yabo on Rosh Hodesh. The reason for this distinction is the Halachic principle of, "En Mekadeshin Et Ha’hodesh Ba’layla." This means that the Sanhedrin (High Rabbinical Court in Jerusalem) would convene to declare the onset of the new month only by day, and not during the nighttime hours. By the same token, the mention of Rosh Hodesh in the Amida at nighttime, in the Arbit service, is not critical for fulfilling the prayer obligation. Hence, one who forgets to recite Ya’ale Ve’yabo in the Arbit service on Rosh Hodesh does not repeat the Amida.
If a person remembers during Arbit on Rosh Hodesh that he forgot Ya’ale Ve’yabo, and he remembers later during the Amida, does he go back to recite Ya’ale Ve’yabo?
The Halacha in such a case is that he does not return to recite Ya’ale Ve’yabo unless he remembered before reciting the phrase, "Baruch Atah Hashem" in the Beracha of "Ha’mahazir Shechinato Le’siyon." Once he recited God’s Name in that Beracha, he no longer has the opportunity to recite Ya’ale Ve’yabo. Even if he remembered immediately after reciting the words, "Baruch Ata Hashem," he should not add the words "Lamedeni Hukecha" to save himself from reciting God’s Name in vain. (Normally, if a person realizes a mistake after reciting "Baruch Ata Hashem," he is advised to add the words "Lamedeni Hukecha" so that he would be considered as reciting the verse from Tehillim, "Baruch Ata Hashem Lamedeni Hukecha," and thereby avoid reciting God’s Name in vain.) This solution is used in the Amida only to avoid mistakes that would invalidate the Amida. In this case, as noted, the Amida is valid even if one omits Ya’ale Ve’yabo, and therefore one does not recite "Lamedeni Hukecha" if he remembers his mistake after reciting "Baruch Ata Hashem."
This applies only to Arbit. When it comes to Shaharit and Minha, however, one who forgets Ya’ale Ve’yabo on Rosh Hodesh must repeat the Amida. Therefore, if a person realizes his mistake immediately after reciting the words, "Baruch Ata Hashem" during Shaharit or Minha, then he should recite the words "Lamedeni Hukecha" at that point, and then return to Ya’ale Ve’yabo. If he recited his mistake (during Shaharit or Minha) after concluding the Beracha, "Ha’mahazir Shechinato Le’siyon," but before beginning "Modim," he should recite Ya’ale Ve’yabo immediately at that point, and then proceed with "Modim." If he remembered his mistake after he began "Modim," then he must return to "Reseh" and continue from there.
If one realized his mistake at the end of the Amida, after he recited the verse of "Yiheyu Le’razon Imreh Fi" for the second time, then he must return to the beginning of the Amida. Once a person recites this verse for the second time, he is considered as having completed the Amida, and it is therefore too late to correct his mistake. At this point, he must begin a new Amida. The exception to this rule is a case of a person who ordinarily adds personal prayers after reciting "Yiheyu Le’rason" for the second time, before reciting "Ose Shalom." For such a person, the Amida is not considered complete until he recites his personal supplications. Therefore, so long as he has not completed those supplications, he may still return to "Reseh" rather than begin a new Amida. Once a person begins reciting "Ose Shalom," however, then he must certainly begin a new Amida.
Summary: One who forgets to recite Ya’ale Ve’yabo during Arbit on Rosh Hodesh does not repeat the Amida. If this happened during Shaharit or Minha, however, he must repeat the Amida. If he realized his mistake after reciting "Baruch Ata Hashem" before reciting "Ha’mahazir Shechinato," then he should recite the words "Lamedeni Hukecha" and go back to Ya’ale Ve’yabo. If he realized after he recited "Ha’mahazir Shechinato Le’siyon" but before "Modim," he inserts Ya’ale Ve’yabo at that point. If he realized after he began "Modim," then he returns to "Reseh." If he realized only after he recited the verse "Yiheyu Le’rason" for the second time, he must start a new Amida.
