Q. Can one wash and eat supper for Rosh Chodesh Seuda or does it have to be done during the day?
A. Poskim disagree. Mishna Berura (419: 2) that for the purpose of the food or meal added for the honor of Rosh Chodesh, it suffices if it is done during the day, and there is no need to do so also at night.
Darkei Teshuva and Nimukei Orach Chaim (419) assert that the seuda must be by day and not at night, since we are not Mekadesh the Chodesh at night.
However, Aruch Laner (Sanhedrin 70b) maintains that according to the Magen Avraham, one complies also if he eats the seuda at night. While Yaskil Avdi (7: p. 294) opines that even at night (leil Rosh Chodesh) one should add some extra food in honor of Rosh Chodesh, as we do in Purim.
Yosef Ometz (692) chastises the ones that eat a seuda on the night after Rosh Chodesh already ended, since if they did not do so during the day, it is an embarrassment for the honor of Rosh Chodesh.
Horav Shlomo Miller’s Shlit’a opinion is to follow the ruling of the Mishna Berura above quoted.
Rabbi A. Bartfeld as advised By Horav Shlomo Miller and Horav Aharon Miller Shlit’a