"Are you going to the party on Wednesday?"
Translation:Dych chi'n mynd i'r parti dydd Mercher?
This exercise accepts both '... i'r parti dydd Mercher' and '... i'r parti ddydd Mercher'. Surely one of these is incorrect?
I think it's a bit like how some people say "I'm going to the party Wednesday" and some "I'm going to the party on Wednesday" - neither is wrong, though any given person may use one form more frequently than the other.
Similarly with dydd Mercher "Wednesday" versus ddydd Mercher "on Wednesday", with the grammatical lenition signifying the use as an adverb.