It should be ddydd Gwener as it is being used as an adverb 'on Friday', but not everybody sticks to that.
- dydd Gwener - Friday
- ddydd Gwener - on Friday (because it is being used as an adverb)
- ddydd Gwener nesaf - on next Friday (as in 'I'll see you next Friday')
- ar ddydd Gwener y degfed o Fedi - on Friday the 20th of September (a particular 'named' Friday)
- ar ddydd Gwener - on Fridays (as in a usual routine) (again, not everybody uses this one either)
- bob dydd Gwener - on every Friday (pob is mutated as this phrase is an adverbial one again) (no mutation after pob)