observance has 7 distinct synonyms spread across 4 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is nonobservance.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the act of observing; taking a patient look
a formal event performed on a special occasion
“a ceremony commemorating Pearl Harbor”
the act of noticing or paying attention
“he escaped the notice of the police”
conformity with law or custom or practice etc.
Synonyms, antonyms and the broader and narrower relations for observance come from Princeton University’s WordNet. Full sources and licences.