The English phrase "on Sundays" is usually written ar an Domhnach, using the singular form of the nominative, but the FGB offers plurals for Monday, Tuesday Wednesday Saturday and Sunday.
Luanta - Mondays
Máirteanna - Tuesdays
Céadaoineacha - Wednesdays
Sathairn - Saturdays
Domhnaí - Sundays
(5 different plurals, 5 different plural endings!)
There is a plural for the noun aoine, meaning a "fasting", (aointe- "fastings"), which is where Aoine - "Friday" comes from, but the entry doesn't make it clear that that applies to Aoine the day.
And the entry for Déardaoin simply doesn't list a plural.