Simply saying Iau and Gwener when listing days is incorrect? Can 'Dydd Mercher, Iau, Gwener' be a correct way of saying such things or all days must have 'dydd' in lists? Thank you for the answers.
You need 'dydd' each time to qualify the word because on their own they mean planets:-
'Dydd Mercher, Iau, Gwener' = Wednesday, Jupiter, Venus.
Dydd Mercher, dydd Iau, dydd Gwener = Wednesday, Thursday, Friday