happening (n.) Look up happening at Dictionary.com
mid-15c., "chance, luck," verbal noun from happen (v.); meaning "an occurrence" is 1550s. Sense of "spontaneous event or display" is from 1959 in the argot of artists. Happenings "events" was noted by Fowler as a vogue word from c. 1905.
happening (adj.) Look up happening at Dictionary.com
1520s, "occurring," present participle adjective from happen (v.).