"Saint David's Day"
Translation:Dydd Gŵyl Dewi
As already mentioned the above is universal in Wales. Any other version is a poor literal translation.
I know, this was the only term I knew used when I was in school, then I moved away and when I came back it was Dydd Gwyl! Ha ha ha
No I wouldn't say so. I'd say that "Dydd Gŵyl Dewi" is and always has been the grammatically correct form and "Dydd Dewi Sant" occured when a bilingual speaker simply translated the English "Saint David's Day" more directly into Welsh.