"Church festival observed annually in memory of the birth of Christ," 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, "to celebrate Christmas," 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 were first designed 1843, popular by 1860s; the phrase Christmas-card was in use by 1850. Christmas-present is from 1769. Christmas Eve is Middle English Cristenmesse Even (c. 1300).