last day has 12 distinct synonyms spread across 1 of its 1 sense. 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
(New Testament) day at the end of time following Armageddon when God will decree the fates of all individual humans according to the good and evil of their earthly lives
Synonyms, antonyms and the broader and narrower relations for last day come from Princeton University’s WordNet. Full sources and licences.