wedding ceremony vs observance

wedding ceremony and observance both are nouns.

Word	Noun	Adjective	Verb	Adverb
wedding ceremony	Yes	No	No	No
observance	Yes	No	No	No

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

wedding ceremony: the social event at which the ceremony of marriage is performed
observance: a formal event performed on a special occasion

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

wedding ceremony (noun)	observance (noun)
the social event at which the ceremony of marriage is performed	the act of observing; taking a patient look
	conformity with law or custom or practice etc.
	the act of noticing or paying attention
	a formal event performed on a special occasion