What is the X in X-mas?
early 13c., "conditions surrounding and accompanying an event," from Old French circonstance "circumstance, situation," also literally, "outskirts" (13c., Modern French circonstance), from Latin circumstantia "surrounding condition," neuter plural of circumstans (genitive circumstantis), present participle of circumstare "stand around, surround, encompass, occupy, take possession of" from circum "around" (see circum-) + stare "to stand" from PIE root *sta- "to stand" (see stet). The Latin word is a loan-translation of Greek peristasis.
Meaning "a person's surroundings, environment" is from mid-14c. Meaning "a detail" is from c.1300; sense of "that which is non-essential" is from 1590s. Obsolete sense of "formality about an important event" (late 14c.) lingers in Shakespeare's phrase pomp and circumstance ("Othello" III, iii).