Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Surfeit is the father of much fast.   --Shak.

   2. Voluntary abstinence from food, for a space of time, as a
      spiritual discipline, or as a token of religious
      humiliation.

   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.

   {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.

Source : WordNet®

fast day
     n : a day designated for fasting