"dydd Sadwrn yma" is "this Saturday", even without a definite article before it?
Yes. Note the mutation of ddydd... because it is being used as an adverb.
I'm just wondering what my indication is that "yma" is being used for "this", rather than "here".
Ah. Reading it again I suppose it could be either:
Intuitively, the former sounds more natural. '...here on Saturday' (...yma ddydd Sadwrn) sounds better for the second meaning.