actually, they weren't far from wrong, but without knowing. Sadwyn is a variant of Sadwen, which is the name of a saint, also known as Sadwrn and
Saturninus. Dyll Sadwrn is Welsh for 'saturday' and so all comes together. However, the name
Saturn is of unknown meaning, but Sadwyn, Sadwen and Sadwrn are the Welsh forms of
Saturn. I hope this is satisfying enough for you.