Childermas

[chil-der-muh s] /ˈtʃɪl dər məs/
noun, Chiefly British
1.
Holy Innocents' Day.
Origin
before 1000; Middle English chyldermasse, equivalent to Old English cildra (genitive plural of cild child) + mæsse Mass
British Dictionary definitions for Childermas

childermas

/ˈtʃɪldəˌmæs/
noun
1.
(archaic) Holy Innocents Day, Dec 28
Word Origin
Old English cylda-mæsse, from cildra, genitive plural of child, + mæsseMass
Word Origin and History for Childermas
n.

"festival of the Holy Innocents" (Dec. 28), late Old English *cildramæsse (c.1000), from obsolete plural of child (q.v.) + mass (n.2).