a prefix used freely in English and meaning "not, lack of," or "sham," giving a negative sense to any word, 14c., from Anglo-French noun-, from Old French non-, from Latin non "not, by no means, not at all, not a," from Old Latin noenum "not one" (*ne oinom, from PIE root *ne- "not" + PIE root *oi-no- "one, unique"). In some cases perhaps from Middle English non "not" (adj.), from Old English nan (see not). "It differs from un- in that it denotes mere negation or absence of the thing or quality, while un- often denotes the opposite of the thing or quality" [Century Dictionary].
1570s, "the consequence of anything" (as in in the event that); 1580s, "that which happens;" from French event, from Latin eventus "occurrence, accident, event, fortune, fate, lot, issue," from past participle stem of evenire "to come out, happen, result," from assimilated form of ex- "out" (see ex-) + venire "to come" (from a suffixed form of PIE root *gwa- "to go, come"). Meaning "a contest or single proceeding in a public sport" is from 1865. Events as "the course of events" is attested from 1842. Event horizon in astrophysics is from 1969.
<a href="https://www.etymonline.com/word/non-event">Etymology of non-event by etymonline</a>
Harper, D. (n.d.). Etymology of non-event. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/non-event