What is the X in X-mas?
word-forming element meaning "before, in front of; previous, existing beforehand; introductory to," from Latin ante (prep. and adv.) "before, in front of, opposite," used in combinations, from PIE *anti "facing opposite, near, in front of, before" (see ante).
Prior to; earlier: antenatal.
In front of; before: antebrachium.