Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Advent \Ad`vent\, n. [L. adventus, fr. advenire, adventum: cf.
   F. avent. See {Advene}.]
   1. (Eccl.) The period including the four Sundays before
      Christmas.

   {Advent Sunday} (Eccl.), the first Sunday in the season of
      Advent, being always the nearest Sunday to the feast of
      St. Andrew (Now. 30). --Shipley.

   2. The first or the expected second coming of Christ.

   3. Coming; any important arrival; approach.

            Death's dreadful advent.              --Young.

            Expecting still his advent home.      --Tennyson.

Source : WordNet®

Advent Sunday
     n : the first of the four Sundays during Advent