Siman 424

One mentions "Ya'aleh VeYavo" in Birkat HaMazon, before "Uveneh Yerushalayim." And if he did not say it – he is not required to return. And it is not similar to prayer, which is an obligation. And it is also not similar to "Retzei VeHachalitzenu" on Shabbat, where one is required to return, because on Shabbat it is an obligation to eat bread, which is not the case on Rosh Chodesh: for although it is forbidden to fast on it, nevertheless, it is not an obligation to eat bread. And since it is optional, for if he wishes he eats, and if he wishes he does not eat – therefore he is not required to return. And it is not similar to Birkat HaMazon itself, where if he erred and forgot a blessing he always returns – this is because the Torah obligation is "And you shall eat and be satisfied and you shall bless." But the obligation of mentioning Rosh Chodesh in Birkat HaMazon – it is a rabbinic obligation, and the sages did not ordain to return except in a place where it is an obligation to eat bread. And nevertheless, if he remembered before he began "HaTov VeHaMeitiv" – he says with the Name "Blessed is He who gave Roshei Chodashim to His people Israel for a remembrance." And above in siman 188 this was explained at length, see there.

It is stated in Masekhet Soferim, chapter nineteen (halacha 9): On Rosh Chodesh, the groups of elders, and of the ballutin (great men), and of the students would sit from the time of mincha and onwards, until the sun would set. And one must say in the blessing over the wine: Borei pri hagafen. Blessed are You, Hashem our G-d, King of the universe, Who in the circle raised His seekers, taught them and instructed them in times, good months, the moon, a complete white light, from which He appointed the understanding ones, the arrangers of seasons; our Rock balanced the ends of moments, through which He established those months and festivals. As it is written: "He made the moon for festivals, the sun knows its coming." And he says: "For as the new heavens..." and concludes: "Blessed are You, Hashem, Who sanctifies Israel and the Rashei Chodashim." And he says: "Give thanks to Hashem for He is good. On this day in Jerusalem may we all be glad and joyful, may Elijah the prophet quickly come to us, may the King Messiah sprout in our days, and in the building of the Beit HaMikdash may joys increase." And the people shall answer and say "Amen! May good tidings increase in Israel, may good days increase in Israel, may students of Torah increase in Israel! The month is sanctified, sanctified on Rosh Chodesh! Sanctified in its time, sanctified in its intercalation! Sanctified in the Torah, sanctified in the halacha! Sanctified in the upper realms, sanctified in the lower realms! Sanctified in the Land of Israel, sanctified in Zion! Sanctified in Jerusalem, sanctified in all places of Israel! Sanctified by the mouths of our teachers, sanctified in the house of assembly! Give thanks to Hashem for He is good, and may you all be blessed, blessed..." Thus far his words. And it appears that this was at the time of the sanctification of the month; happy is the eye that saw all these. And may Hashem merit us to see all these quickly in our days, Amen.

In the previous siman, we wondered why Masechet Soferim did not count the musaf song of Rosh Chodesh. It appears to me that there, at the beginning of the nineteenth chapter, it is stated: And on Rosh Hashanah one says: "All nations clap hands." On Yom Kippur: "Bless the Lord, O my soul" and "Out of the depths I have called You." On the festival of Sukkot: "In Judah is God known..." See there. And it is not understood: what is the connection of "Bless the Lord, O my soul" to Yom Kippur (the Gra erased this)? And furthermore regarding its beginning, where it says: And on Rosh Hashanah. Therefore, it appears to me that it must be said as follows: On Rosh Chodesh "Bless the Lord, O my soul," and on Rosh Hashanah one says "All nations," and on Yom Kippur "Out of the depths...". And according to this, it correctly mentioned the musaf song of Rosh Chodesh.