What is the X in X-mas?
"the Big Dipper;" Middle English septentrioun (1530s in reference to the star pattern; late 14c. as "the North," and septentrional "northern," in reference to the sky, is attested from late 14c.), from Latin septentriones, septemtriones (plural) "the Great Bear, the seven stars of the Big Dipper;" also figuratively "the northern regions, the North;" literally "seven plow oxen," from septem "seven" (see seven) + trio (genitive triones) "plow ox," from stem of terere (past participle tritus) "to rub" (see throw (v.)). Cf. also Charles's Wain.