Shouldn't this be Dw i'n gwisgo gwisg ysgol ar ddydd Gwener
or Dw i'n gwisgo gwisg ysgol ddydd Gwener ?
No since the "ar" has been lost it doesn't have to mutate. (it can but it doesn't have to).
Why does it lose the 'ar'? This is the first time I've been asked about 'on a day' so I included it and was told that I was wrong.
It should be accepted with the "ar" at least that's how I've always used it, but it is pretty common to drop it.
ar causes soft mutation, so dydd becomes ddydd, because of this the ar can be dropped leaving the mutation to do the job of conveying "on".
I put "Dydd Gwener dw i'n gwisgo gwisg ysgol." because I sometimes saw the day at the beginning of a sentence. Is that wrong? I'm a bit confused where the time should go.
The time can go at the start or the end, like English. But in this sentence it's at the end.