formality vs observance

formality and observance both are nouns.

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

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

Other hypernyms of formality include ceremonial, ceremonial occasion, ceremony.

formality (noun)	observance (noun)
compliance with formal rules	the act of observing; taking a patient look
a manner that strictly observes all forms and ceremonies	conformity with law or custom or practice etc.
a requirement of etiquette or custom	the act of noticing or paying attention
	a formal event performed on a special occasion