Dydd Santes Dwynwen, literally meaning 'Day of Saint Dwynwen' in Welsh, is considered to be the Welsh equivalent to Valentine's Day and is celebrated on 25 January every year. It celebrates Dwynwen, the Welsh Saint of love. 25 January is the feast day of Saint Dwynwen.
Much of Welsh history is based on stories and songs as it was considered a virtue to not write these tales down, but to instead pass them down by word of mouth. As such, the original tale has become mixed with elements of folktales and Celtic stories.
This version of the story is generally told to younger children, usually in primary school or nursery. It is generally considered the most appropriate for children.
Dwynwen was the beautiful daughter of Brychan Brycheiniog, who was said to have had eleven sons and twenty-four daughters (although these figures vary greatly, to the extent of suggesting he had over fifty children). She met and fell madly in love with a man called Maelon, and he reciprocated her feelings. She asked her father if she could marry Maelon, but Brychan disliked Maelon and refused to give his permission. Maelon begged, as did Dwynwen, but Brychan would not relent and Maelon was forced to leave. Dwynwen was so upset that she ran into the forests. There, she met with an angel in a dream who granted her the position of the Saint of Love.
Dwynwen did meet a man named Maelon, and they fell in love. However, Dwynwen disliked Maelon's attitude towards sleeping together, as she wished to keep her virtue until after marriage but he wanted them to sleep together. She told him this, and enraged Maelon so much that he attacked and raped her. Dwynwen fled to the woods, distraught. There an angel gave her a magic potion that cooled her love for Maelon (it in fact cooled it too much, as he was turned into a block of ice). It also gave her three wishes. Dwynwen wished that she would never marry, and that she would become the patron saint of lovers to console others through sadness and love. She used the last wish (although some sources say this was her first) to get Maelon unfrozen.
Dwynwen fell in love with a prince called Maelon Dafodrill, but unfortunately her father had already arranged that she should marry someone else. Dwynwen was so upset that she could not marry Maelon that she begged God to make her forget him. After falling asleep, Dwynwen was visited by an angel, who appeared carrying a sweet potion designed to erase all memory of Maelon and turn him into a block of ice. God then gave three wishes to Dwynwen. Her first wish was that Maelon be thawed; her second that God meet the hopes and dreams of true lovers; and third, that she should never marry. 
Dwynwen became a nun, fulfilling her wish to never marry, and left for the island of Anglesey to build a Church. This is referred to as Llanddwynwen, literally meaning 'Church of Dwynwen'.