periodic event vs occurrence

periodic event and occurrence both are nouns.

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

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

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

periodic event (noun)	occurrence (noun)
an event that recurs at intervals	an event that happens
	an instance of something occurring