They are very close to the arthurian "Lady of the Lake" character, and in order to obtain their favours one should be dressed in a blue suite or dress and place at the edge of the bank a tasty bread, being the most likely dates to do it on February 19, 20 and 21.
The Gwragedd Annwn are the elfin dames who dwell under the water. I find no resemblance in the Welsh fairy to our familiar mermaid, beyond the watery abode, and the sometimes winning ways. The Gwragedd Annwn are not fishy of aspect, nor do they dwell in the sea. Their haunt is the lakes and rivers, but especially the wild and lonely lakes upon the mountain heights. These romantic sheets are surrounded with numberless superstitions, which will be further treated of. In the realm of faerie they serve as avenues of communication between this world and the lower one of annwn, the shadowy domain presided over by Gwyn ap Nudd, king of the fairies. These lovely creatures are known to choose mortal men as their husbands. This sub-aqueous realm is peopled by those children of mystery termed Plant Annwn, and the belief is current among the inhabitants of the Welsh mountains that the Gwragedd Annwn still occasionally visit this upper world of ours. ['Archaeologia Cambrensis,' 2nd Se., iv., 253] The only reference to Welsh mermaids I have either read or heard is contained in Drayton's account of the Battle of Agincourt. There it is mentioned, among the armorial ensigns of the counties of Wales:
Came with a mermaid sitting on a rock
[There is in 'Cymru Fu' a mermaid story, but its mermaid feature is apparently a modern embellishment of a real incident, and without value here.]
A Legend of Crumlyn Lake
Crumlyn Lake, near the quaint village of Briton Ferry, is one of the many in Wales which are a resort of the elfin dames. It is also believed that a large town lies swallowed up there, and that the Gwragedd Annwn have turned the submerged walls to use as the superstructure of their fairy palaces. Some claim to have seen the towers of beautiful castles lifting their battlements beneath the surface of the dark waters, and fairy bells are at times heard ringing from these towers.
The way the elfin dames first came to dwell there was this: A long, ay, a very long time ago, St. Patrick came over from Ireland on a visit to St. David of Wales, just to say 'Sut yr y'ch chwi?' (How d'ye do?); and as they were strolling by this lake conversing on religious topics in a friendly manner, some Welsh people who had ascertained that it was St. Patrick, and being angry at him for leaving Cambria for Erin, began to abuse him in the Welsh language, his native tongue.
Of course such an insult could not go unpunished, and St. Patrick caused his villifiers to be transformed into fishes; but some of them being females, were converted into fairies instead. It is also related that the sun, on account of this insolence to so holy a man, never shed its life-giving rays upon the dark waters of this picturesque lake, except during one week of the year. This legend and these magical details are equally well accredited to various other lakes, among them Llyn Barfog, near Aberdovey, the town whose 'bells' are celebrated in immortal song.