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).