"They want to swim Friday."
Translation:Maen nhw eisiau nofio dydd Gwener.
Is "ar ddydd Gwener" not also acceptable? It says it is wrong but it is the form I mostly use and I often hear.
ar ddydd Gwener usually means on Fridays, unless it is clear that a particular Friday is being discussed:
- Dw i'n mynd i nofio ar ddydd Gwener - I go swimming on Fridays
- Dw i'n mynd i nofio bob dydd Gwener - I go swimming every Friday
- Dw i'n mynd i nofi (ar) ddydd Gwener nesaf - I am going swimming next Friday - ar optional
- Dw i'n mynd i nofio ddydd Gwener - I am going swimming on Friday - but a 'next' could be implied from the rest of the conversation, perhaps.
In general spoken use, though, the context of a conversation makes a lot of these small differences less relevant.