What is the X in X-mas?
early 15c., from Latin intermissionem (nominative intermissio) "interruption," noun of action from past participle stem of intermittere "to leave off," from inter- "between" (see inter-) + mittere "let go, send" (see mission).
Intermission is used in U.S. for what we call an interval (in a musical or dramatic performance). Under the influence of LOVE OF THE LONG WORD, it is beginning to infiltrate here and should be repelled; our own word does very well. [H.W. Fowler, "Modern English Usage," 1926]