The Collaborative International Dictionary of English v.0.48:

Sunday \Sun"day\, n. [AS. sunnandaeg; sunne, gen. sunnan, the
   sun + daeg 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.
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   Advent Sunday, Low Sunday, Passion Sunday, etc. See
      under Advent, Low, etc.
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   Syn: See Sabbath.
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The Collaborative International Dictionary of English v.0.48:

Advent \Ad`vent\, n. [L. adventus, fr. advenire, adventum: cf.
   F. avent. See Advene.]
   1. (Eccl.) The period including the four Sundays before
      Christmas.
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   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.
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   2. The first or the expected second coming of Christ.
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   3. Coming; any important arrival; approach.
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            Death's dreadful advent.              --Young.
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            Expecting still his advent home.      --Tennyson.
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WordNet (r) 3.0 (2006):

Advent Sunday
    n 1: the first of the four Sundays during Advent