How do you say Happy St. David's day in Welsh?

Answer 1

Dydd Gŵyl Dewi Sant hapus

St David is the patron saint of Wales. And should you wish to convey your best wishes for that day in Welsh (also the language of a certain Maelwyn, who later found fame as St. Patrick, depending on who you ask) there are several options, but the most common is probably Dydd gwyl Dewi Sant hapus.
You would say Dydd gwyl Dewi Sant hapus, which is pronounced (approximately) 'Deeth (as in the, /ð/) gooyl Dewi Sant Hapiss'. Hope this helps!