Shulchan Arukh, Orach Chayim Siman 419
Can copy this into an AI, or connect the whole library to your assistant. For deeper learning, you can directly use the original seforim, sites, and apps.
ובו סעיף אחד
מצוה להרבות בסעודת ר"ח:
Mishna Berura Yomi
It is a Mitzvah to feast a lot on Rosh Chodesh.