[1] : 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.
   1913 webster

   advent sunday, low sunday, passion sunday, etc. see
      under advent, low, etc.
      1913 webster

   syn: see sabbath.
        1913 webster
see also:
sun day advent sunday low sunday passion sunday advent 
low sabbath 

[2] : 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.
      1913 webster

   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.
      1913 webster

   2. the first or the expected second coming of christ.
      1913 webster

   3. coming; any important arrival; approach.
      1913 webster

            death's dreadful advent.              --young.
      1913 webster

            expecting still his advent home.      --tennyson.
      1913 webster
see also:
advene advent sunday 

[3] : WordNet (r) 2.0

advent sunday
     n : the first of the four sundays during advent

Sunday >> S Words
Sunday, definition of term: Sunday
sunday_pag1.html

Advent >> A Words
Advent, definition of term: Advent
advent_pag1.html

Trinity Sunday >> T Words
Trinity Sunday, definition of term: Trinity Sunday
trinity+sunday_pag1.html

Palm Sunday >> P Words
Palm Sunday, definition of term: Palm Sunday
palm+sunday_pag1.html

Low Sunday >> L Words
Low Sunday, definition of term: Low Sunday
low+sunday_pag1.html

Passion Sunday >> P Words
Passion Sunday, definition of term: Passion Sunday
passion+sunday_pag1.html