observance vs occasion

observance and occasion both are nouns.

Word	Noun	Adjective	Verb	Adverb
observance	Yes	No	No	No
occasion	Yes	No	Yes	No

As nouns, occasion is a hypernym of observance; that is, occasion is a word with a broader meaning than observance:

Other hypernyms of observance include affair, function, social affair, social function, social gathering, social occasion.

observance (noun)	occasion (noun)
the act of observing; taking a patient look	an event that occurs at a critical time
conformity with law or custom or practice etc.	a formal or official social gathering or ceremony of people
the act of noticing or paying attention	a rational motive for a belief or action
a formal event performed on a special occasion	an opportunity to do something
	the time of a particular event

observance (verb)	occasion (verb)
	give occasion to