WordNet (r) 3.0 (2006):

holy day
    n 1: a day specified for religious observance [syn: religious
         holiday, holy day]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "holy day":
   High Holiday, High Holy Day, Sabbath, Sunday, bank holiday,
   church calendar, church feast, day of festivities, day off,
   ecclesiastical calendar, fast, feast, feast day, festival day,
   fete day, fixed feast, gala day, high day, holiday, holytide,
   legal holiday, red-letter day