What is the X in X-mas?
c.1400, "action of establishing or founding (a system of government, a religious order, etc.)," from Old French institucion "foundation; thing established," from Latin institutionem (nominative institutio) "disposition, arrangement; instruction, education," noun of state from institutus (see institute). Meaning "established law or practice" is from 1550s. Meaning "establishment or organization for the promotion of some charity" is from 1707.