Correction:
Mechaber in Orach Chaim 570-3 says if one vows not to eat on Shabbos or Yom Tov, it does not take effect because we have a Torah obligation to eat.In the times of Chazal he would get malkos for an unnecessary vow. If he vowed not to eat on Chanukah or Purim it takes effect because the eating then is only D’Rabanan. The mechaber continues that the same applies to Rosh Chodesh and Erev Yom Kippur. But he syas that the Rambam argues and says that they are dioraysa like Shabbos and Yom Tov so the vow would not take effect.
Sorry, I should have checked the details before posting.