day has 10 distinct synonyms spread across 4 of its 10 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 night.
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.
time for Earth to make a complete rotation on its axis
“two days later they left”
some point or period in time
“it should arrive any day now”
a day assigned to a particular purpose or observance
“Mother's Day”
the time after sunrise and before sunset while it is light outside
“the dawn turned night into day”
an era of existence or influence
“in the day of the dinosaurs”
the period of time taken by a particular planet (e.g. Mars) to make a complete rotation on its axis
“how long is a day on Jupiter?”
the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day
a period of opportunity
“he deserves his day in court”
United States writer best known for his autobiographical works (1874-1935)
Synonyms, antonyms and the broader and narrower relations for day come from Princeton University’s WordNet. Full sources and licences.