Written as one word from mid-14c. As a verb, "to celebrate Christmas," from 1590s. Father Christmas is attested in a carol attributed to Richard Smart, Rector of Plymtree (Devon) from 1435-77. Christmas-tree in the modern sense is attested by 1835 in American English, rendering German Weihnachtsbaum. Christmas cards were first designed in 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).
also figgey, 1540s "sweet" (as figs are), from fig (n.1) + -y (2). From 1846 (in a book of Cornish words) as "full of figs or raisins." The figgy pudding of the Christmas carol is a dish of dried figs stewed in wine that dates back to the Middle Ages but was more often associated with Lent than Christmas.
early 12c., Nativite, "feast-day celebrating the birth of Christ, Christmas," from Old French nativité "birth, origin, descent; birthday; Christmas" (12c.), from Late Latin nativitatem (nominative nativitas) "birth," from Latin nativus "born, native" (see native (adj.)). Late Old English had nativiteð, from earlier Old French nativited. From late 14c. as "fact of being born; circumstances attending one's birth."
also embergoose, "loon," 1744, from Norwegian emmer-gaas, perhaps so called from its appearing on the coast in the ember-days before Christmas.