Christmas (n.)
late Old English Cristes mæsse, from Christ (and retaining the original vowel sound) + mass (n.2).

Written as one word from mid-14c. As a verb from 1590s. Father Christmas first attested in a carol attributed to Richard Smart, Rector of Plymtree (Devon) from 1435-77. Christmas tree in modern sense first attested 1835 in American English, from German Weihnachtsbaum. Christmas cards first designed 1843, popular by 1860s. Christmas Eve is Middle English Cristenmesse Even (c.1300).