The Collaborative International Dictionary of English v.0.48:

Ember \Em"ber\, a. [OE. ymber, AS. ymbren, ymbryne, prop.,
   running around, circuit; ymbe around + ryne a running, fr.
   rinnan to run. See Amb-, and Run.]
   Making a circuit of the year of the seasons; recurring in
   each quarter of the year; as, ember fasts.
   [1913 Webster]

   Ember days (R. C. & Eng. Ch.), days set apart for fasting
      and prayer in each of the four seasons of the year. The
      Council of Placentia [A. D. 1095] appointed for ember days
      the Wednesday, Friday, and Saturday after the first Sunday
      in Lent, Whitsuntide, the 14th of September, and the 13th
      of December. The weeks in which these days fall are called
      ember weeks.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "Ember days":
   Advent, Allhallowmas, Allhallows, Allhallowtide, Annunciation,
   Annunciation Day, Ascension Day, Ash Wednesday, Candlemas,
   Candlemas Day, Carnival, Christmas, Corpus Christi, Easter,
   Easter Monday, Easter Saturday, Easter Sunday, Eastertide,
   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