From Longman Dictionary of Contemporary Englishfast dayˈfast ˌday noun [countable] RREATa day when you do not eat any food, especially for religious reasons
Examples from the Corpus
fast day• N.B. It does no harm to include a fast day in the feeding regime.• The date commemorates the fall of both Temple5-It is a fast day and an occasion of double mourning.• It is a fast day and an occasion of double mourning. 