'Advent Sunday' definitions:

Definition of 'Advent Sunday'

(from WordNet)
noun
The first of the four Sundays during Advent

Definition of 'Advent Sunday'

From: GCIDE
  • 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]