WordNet records no synonym and no antonym for calendar. It is the only member of each of its synsets, 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 system of timekeeping that defines the beginning and length and divisions of the year
a list or register of events (appointments or social events or court cases etc)
“I have you on my calendar for next Monday”
Synonyms, antonyms and the broader and narrower relations for calendar come from Princeton University’s WordNet. Full sources and licences.
a tabular array of the days (usually for one year)