On Rosh Hodesh we add Ya’ale Ve’yabo in the Amida prayers at Arbit, Shaharit and Minha, and in Birkat Ha’mazon. One who forgot to recite Ya’aleh Ve’yabo in Birkat Ha’mazon on Rosh Hodesh, or in the Amida of Arbit, does not repeat the Birkat Ha’mazon or the Amida. If, however, one forgot to recite Ya’ale Ve’yabo in the Amida at Shaharit or Minha, he must repeat the Amida.
It must be emphasized, however, that although one does not repeat Birkat Ha’mazon or the Amida of Arbit if he forgot Ya’ale Ve’yabo, this does not mean that the recitation is not important. It is told that a student of the Arizal (Rav Yishak Luria of Safed, 1534-1572) once forgot Ya’aleh Ve’yabo on Rosh Hodesh, and that night he dreamt the Pasuk, "Netanani Hashem Bi’ydeh Lo Uchal Kum" – "G-d has placed me in the hands of those against whom I cannot arise." The student came to the Arizal and asked what this vision meant, and the Arizal explained that he must have forgotten Ya’aleh Ve’yabo, which has the power to "lift" the Shechina, and Hashem therefore showed this man that he placed G-d in a position where He could not be elevated, as it were.
Likewise, Hacham Bension Abba Shaul (Israel, 1923-1998) used to say that he does not understand how the Halachic authorities could speak about a case of somebody forgetting to recite Ya’aleh Ve’yabo. If somebody is invited to come and speak with a king, he carefully prepares his remarks in advance and ensures to say everything properly. Hacham Bension thus could not understand how somebody could possibly forget a required component of the Amida. On one occasion, a person came with him with a question regarding the forgotten recitation of Ya’aleh Ve’yabo, and Hacham Bension instructed the fellow to bring his question to Rav Elyashiv. He said, "I am sure he never forgot Ya’aleh Ve’yabo in his life, so he is worthy of answering this question."
In fact, a number of Poskim (Rav Haim Palachi and the Hesed La’alafim) write that one who forgets to recite Ya’aleh Ve’yabo on Rosh Hodesh must be anxious and concerned throughout that month.
Thus, although the Halacha is that one who forgets Ya’aleh Ve’yabo at Arbit or Birkat Ha’mazon on Rosh Hodesh does not repeat the Amida or Birkat Ha’mazon, one must ensure not to forget it.
Summary: One who forgets to recite Ya’aleh Ve’yabo in the Amida of Arbit on Rosh Hodesh, or in Birkat Ha’mazon on Rosh Hodesh, does not repeat the Amida or Birkat Ha’mazon, but every effort must be made to avoid such a situation.
