Source : Webster's Revised Unabridged Dictionary (1913)

Sunday \Sun"day\, a.
   Belonging to the Christian Sabbath.

   {Sunday letter}. See {Dominical letter}, under {Dominical}.
      

   {Sunday school}. See under {School}.

Sunday \Sun"day\, n. [AS. sunnand[ae]g; sunne, gen. sunnan, the
   sun + d[ae]g day; akin to D. zondag, G. sonntag; -- so called
   because this day was anciently dedicated to the sun, or to
   its worship. See {Sun}, and {Day}.]
   The first day of the week, -- consecrated among Christians to
   rest from secular employments, and to religious worship; the
   Christian Sabbath; the Lord's Day.

   {Advent Sunday}, {Low Sunday}, {Passion Sunday}, etc. See
      under {Advent}, {Low}, etc.

   Syn: See {Sabbath}.

Source : WordNet®

Sunday
     n : first day of the week; observed as a day of rest and worship
         by most Christians [syn: {Lord's Day}, {Dominicus}, {Sun}]