occurrence vs happening

occurrence and happening both are nouns.

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

As nouns, occurrence and happening are synonyms defined as:

Other synonyms of occurrence include natural event, occurrent.

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