The Collaborative International Dictionary of English v.0.48:

Wednesday \Wednes"day\ (?; 48), n. [OE. wednesdai, wodnesdei,
   AS. W[=o]dnes d[ae]g, i. e., Woden's day (a translation of L.
   dies Mercurii); fr. W[=o]den the highest god of the Teutonic
   peoples, but identified with the Roman god Mercury; akin to
   OS. W[=o]dan, OHG. Wuotan, Icel. O[eth]inn, D. woensdag
   Wednesday, Icel. [=o][eth]insdagr, Dan. & Sw. onsdag. See
   Day, and cf. Woden, Wood, a.]
   The fourth day of the week; the next day after Tuesday.
   [1913 Webster]

   Ash Wednesday. See in the Vocabulary.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ash Wednesday \Ash` Wednes"day\ ([a^]sh` w[e^]nz"d[asl]).
   The first day of Lent; -- so called from a custom in the
   Roman Catholic church of putting ashes, on that day, upon the
   foreheads of penitents.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Ash Wednesday
    n 1: the 7th Wednesday before Easter; the first day of Lent; the
         day following Mardi Gras (`Fat Tuesday'); a day of fasting
         and repentance

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "Ash Wednesday":
   Advent, Allhallowmas, Allhallows, Allhallowtide, Annunciation,
   Annunciation Day, Ascension Day, Candlemas, Candlemas Day,
   Carnival, Christmas, Corpus Christi, Easter, Easter Monday,
   Easter Saturday, Easter Sunday, Eastertide, Ember days, Epiphany,
   Good Friday, Halloween, Hallowmas, Holy Thursday, Holy Week,
   Lady Day, Lammas, Lammas Day, Lammastide, Lent, Lententide,
   Mardi Gras, Martinmas, Maundy Thursday, Michaelmas, Michaelmas Day,
   Michaelmastide, Palm Sunday, Pancake Day, Passion Week, Pentecost,
   Quadragesima, Quadragesima Sunday, Septuagesima, Shrove Tuesday,
   Trinity Sunday, Twelfth-day, Twelfth-tide, Whit-Tuesday,
   White Sunday, Whitmonday, Whitsun, Whitsunday, Whitsuntide,
   Whitweek