What is the X in X-mas?
late 14c., "giving heed," from Latin attentionem (nominative attentio) "attention, attentiveness," noun of action from past participle stem of attendere "mental heeding" (see attend). Used with a remarkable diversity of verbs (e.g. pay, gather, attract, draw, call). As a military cautionary word preparative to giving a command, it is attested from 1792.