observance vs affair

observance and affair both are nouns.

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

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

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

observance (noun)	affair (noun)
the act of observing; taking a patient look	a vaguely specified concern
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 usually secretive or illicit sexual relationship
a formal event performed on a special occasion