'Fast day' definitions:

Definition of 'fast day'

noun

A day designated for fasting

From: GCIDE

Fast \Fast\, n. [OE. faste, fast; cf. AS. f[ae]sten, OHG. fasta, G. faste. See Fast, v. i.]
1. Abstinence from food; omission to take nourishment. [1913 Webster]
Surfeit is the father of much fast. --Shak. [1913 Webster]
2. Voluntary abstinence from food, for a space of time, as a spiritual discipline, or as a token of religious humiliation. [1913 Webster]
3. A time of fasting, whether a day, week, or longer time; a period of abstinence from food or certain kinds of food; as, an annual fast. [1913 Webster]
Fast day, a day appointed for fasting, humiliation, and religious offices as a means of invoking the favor of God.
To break one's fast, to put an end to a period of abstinence by taking food; especially, to take one's morning meal; to breakfast. --Shak. [1913 Webster]