"Mae sêl cist car ddydd Sadwrn yma yn y bore. Hoffet ti fynd?"

Translation: There is a car boot sale this Saturday in the morning. Would you like to go?

[StephenDunscombe]

"dydd Sadwrn yma" is "this Saturday", even without a definite article before it?

[ibisc]

Yes. Note the mutation of ddydd... because it is being used as an adverb.

[StephenDunscombe]

I'm just wondering what my indication is that "yma" is being used for "this", rather than "here".

[ibisc]

Ah. Reading it again I suppose it could be either:

• '...this Saturday', or
• '...on Saturday here'

Intuitively, the former sounds more natural. '...here on Saturday' (...yma ddydd Sadwrn) sounds better for the second meaning.