"Saint Dwynwen's day"
Translation:Dydd Santes Dwynwen
is 'dewi sant' a unique construction for st. david? 'dwynwyn sant' didn't work for me.
Yes, Dewi Sant is particular to him, recognising his particular importance to Wales as 'Dewi the Saint' (if it was translated into English).
Several other historical figures have this sort of honorific title, which is quite often mutated: Dafydd Gam, Rhodri Mawr, Hywel Dda, ... (Dafydd Crookback, Rhodri the Great, Hywel the Good, ...).
Note that Santes is a feminine form, as in Santes Dwynwen.
Yes I believe it is. I can't think of any other saint where we put "Sant/Santes" after the name.