Very obscure patron of the mining village of St Day, near Redruth, Cornwall, possibly to be identified with the Breton Dei or Thei, said to be a monk of Landevennec and abbot. There is no evidence of liturgical cult on either side of the Channel and consequently no feast day. The chapel of St Day was dedicated to the Holy Trinity before 1269.
G. H. Doble, Saint Day (Cornish Saints, no. 32), 1933. Baring-Gould and Fisher, ii. 322–3.