WordNet records no synonym and no antonym for feria. 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 weekday on which no festival or holiday is celebrated
“in the middle ages feria was used with a prefixed ordinal number to designate the day of the week, so `secunda feria' meant Monday, but Sunday and Saturday were always called by their names, Dominicus and Sabbatum, and so feria came to mean an ordinary weekday”
(in Spanish speaking regions) a local festival or fair, usually in honor of some patron saint
Synonyms, antonyms and the broader and narrower relations for feria come from Princeton University’s WordNet. Full sources and licences.