vernal equinox has 2 distinct synonyms spread across 1 of its 2 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 autumnal equinox.
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.
March 21
(astronomy) the equinoctial point that lies in the constellation of Pisces
Synonyms, antonyms and the broader and narrower relations for vernal equinox come from Princeton University’s WordNet. Full sources and licences.