Shulchan Aruch says that it is a mitzvah to increase one’s seudah on every Rosh Chodesh.
When Rosh Chodesh is on Shabbos:
- The Bach suggests making the seudas Rosh Chodesh on Sunday, when Rosh Chodesh is Shabbos.
- Magen Araham suggests making a really l-o-n-g seudah shlishis, l’kovod Rosh Chodesh.
- The prevalent minhag is to add a special food to the seudas Shabbos l’kovod Rosh Chodesh.