Siman 419

The Tur wrote: "It is a mitzvah to increase the meal of Rosh Chodesh. For we learn in Megillah, chapter one (in the Yerushalmi) on the Mishnah that we advance and do not delay: But the meal of Purim and the meal of Rosh Chodesh—we delay and do not advance. That is to say: when Rosh Chodesh falls on Shabbat, since there is anyway the Shabbat meal—we make the Rosh Chodesh meal on Sunday, like Purim that falls on Shabbat. And since it compares it to Purim—learn from this that it is an obligation. Furthermore: for it is equated to a 'Moed' in the verse 'And on the day of your gladness.' And it is written regarding David: 'For there is a family sacrifice for us,' and it was Rosh Chodesh, as it is written 'And it was on the morrow of the second New Moon.' And in the Pesikta it is stated: All of a person's sustenance is fixed for him from Rosh Hashanah, except for what he spends on the expenses of Shabatot, and Yamim Tovim, and Rosh Chodesh, and Chol HaMoed, and what the children take to the house of their teacher. If he adds—they add for him etc.,' thus far the wording of the Tur.

And also in the Rokeach (siman 228) he wrote so. And he further cited the verse, "And it shall come to pass, that from one new moon to another, and from one shabbat to another," and the verse, "Wherefore wilt thou go to him today? It is neither new moon nor shabbat," see there. And so the Kolbo wrote that the meal is for the honor of the day, for it is an atonement for Israel, etc., a remembrance of the meal they would make for the witnesses of the moon in the time of the Beit HaMikdash, etc. Thus far his words. And it appears that his intention is to the meal they would make for the intercalation of the month, as it is found in the Yerushalmi. And so it appears from our Gemara in Sanhedrin, regarding the matter of a stubborn and rebellious son, that if he ate at the intercalation of the month—he does not become a stubborn and rebellious son (70a). And the wonder is great: why have we not practiced at all the meal of Rosh Chodesh? And all the more so when Rosh Chodesh falls on shabbat, that they do not make a meal on Sunday? And perhaps the world took the view because in our Gemara there is no mention of this. For behold, the Pesikta that counted Rosh Chodesh among the expenses of shabbatot and festivals—is not in our Gemara at the beginning of the second chapter of Beitzah. And also: that which is mentioned regarding a stubborn and rebellious son implies that the intercalation of the month has no mitzvah in it, for it teaches: if he ate in a group of a mitzvah, if he ate at the intercalation of the month—learn from this that this is not a mitzvah. And also our teacher, the Beit Yosef, in his great book rejected all the proofs of the Tur, see there. And nevertheless, it is proper to be concerned for this, but what shall we do when the hand of time is hard upon us? And nevertheless, those who are meticulous add some food on Rosh Chodesh, and so on shabbat Rosh Chodesh they add a dish in honor of Rosh Chodesh. (And what the Magen Avraham wrote regarding if his meal extended until the night, regarding the mention of "Retzei" and "Ya'aleh V'Yavo"—we explained above in siman 188.)