Saint David was born in Caerfai, south west Wales into an aristocratic family. He founded a Celtic monastic community at Glyn Rhosyn (The Vale of Roses). His foundation at Glyn Rhosyn became an important Christian shrine. David’s fame as a teacher and his asceticism spread among Celtic Christians, and he helped found about 12 monasteries.
For centuries, 1 March has been a national festival. Saint David was recognised as a national patron saint in the 12th century. Traditional festivities include wearing daffodils and leeks, recognised symbols of Wales and Saint David, respectively, eating traditional Welsh food including cawl and Welsh rarebit, and women wearing traditional Welsh dress. An increasing number of cities and towns across Wales including Cardiff, Swansea and Aberystwyth also put on parades throughout the day.
The date of Saint David’s death is believed to be 1 March 589. His final words to the community of monks were: “Brothers be ye constant. The yoke which with single mind ye have taken, bear ye to the end; and whatsoever ye have seen with me and heard, keep and fulfil.”
Sources:
https://en.wikipedia.org/wiki/Saint_David%27s_Day
St David's Day 2017: everything you need to know about Wales' patron saint". The Daily Telegraph.
"St David (?-589)". BBC News. Archived from the original on 13 March 2017
Jenkins, Simon (2008). Wales: churches, houses, castles. Allen Lane. p. 125.