periodic event vs happening

periodic event and happening both are nouns.

Word	Noun	Adjective	Verb	Adverb
periodic event	Yes	No	No	No
happening	Yes	No	No	No

As nouns, happening is a hypernym of periodic event; that is, happening is a word with a broader meaning than periodic event:

Other hypernyms of periodic event include natural event, occurrence, occurrent.

periodic event (noun)	happening (noun)
an event that recurs at intervals	an event that happens