"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 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).
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).
<a href="https://www.etymonline.com/word/Christmassy">Etymology of Christmassy by etymonline</a>
Harper, D. (n.d.). Etymology of Christmassy. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Christmassy