Translation:prynhawn dydd Sul
When we want to say that we are just doing something on a particular day, we change dydd xxx to ddydd xxx, where the initial d- takes a soft mutation to dd-. (This is because the name of the day is being used as an adverb.)
In 'good Welsh' It is usual to mutate the first element in an adverbial expression such as this one, describing WHEN I am working:
dydd Llun - Monday
Dw i'n gweithio ddydd Llyn - I am working ON Monday
(d)dydd Llyn meaning (on) Monday in this case.
Not everybody does this, however, especially in casual usage.
"ar ddydd Gwener" usually has the meaning "on fridays".
Again, this distinction is not always made in casual Welsh.