observance vs ceremonial occasion

observance and ceremonial occasion both are nouns.

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

As nouns, observance and ceremonial occasion are synonyms defined as:

observance and ceremonial occasion: a formal event performed on a special occasion

Other synonyms of observance include ceremonial, ceremony.

observance (noun)	ceremonial occasion (noun)
the act of observing; taking a patient look	a formal event performed on a special occasion
conformity with law or custom or practice etc.
the act of noticing or paying attention
a formal event performed on a special occasion