Dictionary.net

• chil'd[.e]r-mas-d[=a], n. an anniversary in the Church of England, called also Innocents' Day, held 28th December, to commemorate the slaying of the children by Herod. [CHILD, MASS, and DAY.]

1908 - Chambers's Twentieth Century Dictionary of the English Language
By Thomas Davidson

• n. An anniversary of the church held on the 28th of December; in commemoration of the children of Bethlehem slain by Herod.

1871 - The Cabinet Dictionary of the English Language