happening vs occurrence

happening and occurrence both are nouns.

Word	Noun	Adjective	Verb	Adverb
happening	Yes	No	No	No
occurrence	Yes	No	No	No

As nouns, happening and occurrence are synonyms defined as:

Other synonyms of happening include natural event, occurrent.

happening (noun)	occurrence (noun)
an event that happens	an event that happens
	an instance of something occurring