HAPPENING
happening, occurrence, occurrent, natural event
(noun) an event that happens
Source: WordNet® 3.1
Etymology 1
Verb
happening
present participle of happen
Adjective
happening (comparative more happening, superlative most happening)
(slang, of a place) Busy, lively; vibrant, dynamic; fashionable.
(slang, of a person or product) Trendy, up-to-the-minute.
Etymology 2
Noun
happening (plural happenings)
Something that happens.
A spontaneous or improvised event, especially one that involves audience participation.
Source: Wiktionary
HAPPEN
Hap"pen, v. i. [imp. & p. p. Happened; p. pr. & vb. n. Happening.]
Etym: [OE. happenen, hapnen. See Hap to happen.]
1. To come by chance; to come without previous expectation; to fall
out.
There shall no evil happen to the just. Prov. xii. 21.
2. To take place; to occur.
All these things which had happened. Luke xxiv. 14.
To happen on, to meet with; to fall or light upon. "I have happened
on some other accounts." Graunt.
– To happen in, to make a casual call. [Colloq.]
Source: Webster’s Unabridged Dictionary 1913 Edition