Source : Webster's Revised Unabridged Dictionary (1913)

Childermas day \Chil"dermas day`\ [AS. cildam[ae]sse-d[ae]g;
   cild child +d[ae]g day.] (Eccl.)
   A day (December 28) observed by mass or festival in
   commemoration of the children slain by Herod at Bethlehem; --
   called also {Holy Innocent's Day}.