[1] : The Collaborative International Dictionary of English v.0.48

christmas \christ"mas\, n. christ + mass.
   an annual church festival december 25 and in some states a
   legal holiday, in memory of the birth of christ, often
   celebrated by a particular church service, and also by
   special gifts, greetings, and hospitality.
   1913 webster

   christmas box.
   a a box in which presents are deposited at christmas.
   b a present or small gratuity given to young people and
       servants at christmas; a christmas gift.

   christmas carol, a carol sung at, or suitable for,
      christmas.

   christmas day. same as christmas.

   christmas eve, the evening before christmas.

   christmas fern bot., an evergreen north american fern
      aspidium acrostichoides, which is much used for
      decoration in winter.

   christmas flower, christmas rose, the black hellebore, a
      poisonous plant of the buttercup family, which in southern
      europe often produces beautiful roselike flowers
      midwinter.

   christmas tree, a small evergreen tree, set up indoors, to
      be decorated with bonbons, presents, etc., and illuminated
      on christmas eve.
      1913 webster
see also:
christmas box christmas carol christmas day christmas christmas eve christmas fern 
aspidium acrostichoides christmas flower christmas rose christmas tree 

[2] : WordNet (r) 2.0

christmas eve
     n : the day before christmas syn: dec 24
see also:
dec 24 

Allhallows Eve >> A Words
Allhallows Eve, definition of term: Allhallows Eve
allhallows+eve_pag1.html