WordNet records no synonym and no antonym for sabbath. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
a day of rest and worship: Sunday for most Christians; Saturday for the Jews and a few Christians; Friday for Muslims
Synonyms, antonyms and the broader and narrower relations for sabbath come from Princeton University’s WordNet. Full sources and licences.