"I walk on Tuesdays."
Translation:Siúlaim ar an Máirt.
In another exercise, "On Tuesday or on Wednesday" was translated as "De mairt no de ceadaoin", and "ar an mairt no ar an gceadaoin" was marked wrong.
I don't see what the difference is.
(I don't accept that it's singular/plural, because I would use "I walk on Tuesday" in this context.)