According to Welsh sources collected in the 15th century by John Capgrave and published in the Nova Legenda Angliae, Cenydd was a Breton prince, the son of "King Dihoc" (presumably Deroch II of Domnonée) by an allegedly incestuous relationship with his own daughter.

[2] While the poor girl was pregnant, Deroch was summoned by King Arthur to attend the Christmas festivities being held at his court in Aber Llychwr (Loughor).

A cripple, Cenydd was placed in a cradle made of osiers and cast into the estuary of the River Loughor (a fate that befell several early British saints) and eventually landed on Worm's Head.

In 545, Saint David cured Cenydd while traveling to the Synod of Brefi but he preferred to remain as he was born and prayed for his infirmity to be restored.

Up to the early twentieth century the festival was traditionally marked by the displaying of an effigy of a bird from a pole on the church tower, symbolising the legendary birds who cared for the infant Cenydd, and the consumption of whitepot or 'milked meat' a dish made of flour, milk, sugar and dried fruits, not unlike a rice pudding or bread and butter pudding (see also Cuisine of Gower).