What is the X in X-mas?
late 14c., "freedom from obligations, leisure, release" (from some activity or occupation), from Old French vacation, from Latin vacationem (nominative vacatio) "leisure, a being free from duty," noun of state from past participle stem of vacare "be empty, free, or at leisure" (see vain).
Meanings "state of being unoccupied; process of vacating" are early 15c. Meaning "formal suspension of activity" (in reference to schools, courts, etc.) is recorded from mid-15c. As the U.S. equivalent of what in Britain is called a holiday, it is attested from 1878.
1876, from vacation (n.). Related: Vacationed; vacationing.
Ultraviolet rays; sunshine: catch some UVs on the deck