"Maen nhw eisiau nofio dydd Gwener."

Translation: They want to swim Friday.

[yuvalinbar]

So you just say "I want to swim Friday"?

[ibisc]

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)