The Irish have their St. Patrick's Day (3/17), the Welsh have their St. David's, with parades and other events during the Welsh "national holiday." Apparently 87 percent of the would like to have St Davids be a "bank holiday," but a petition to that effect in 2007 was rejected by Tony Blair. While the Irish has their shamrock, the Welsh have their leek to wear.
We don't know a great deal about David, and while you will find the usual skeptical opinion--"he is so shrouded in legend and overlaid with later hagiography that maybe he never existed"--there is enough historical evidence to believe that he existed, that he taught in Wales, and that he made a marked that lasted and spread. (The earliest surviving reference to him may well be an early 7th-century inscription on a gravestone: "Here lies Idnert, the son of Jacob, who was killed defending the church of holy David from pillage.") You find this pattern of influence throughout the history of the church--Patrick in Ireland, Columba on Iona, Aidan of Lindisfarne, Romanus and Lupicinus of the Jura Mountains, Gall in Switzerland, Boniface in Germany, Genevieve in Paris, and certainly there are modern examples--the Orthodox might point to Herman of Alaska and the influence and fruit of his labor among the native populations.
We see someone who takes the Gospel seriously, gives his or her entire life to living it and spreading it, counting themselves as servants of Jesus Christ, and a previously pagan society some years or generations later has become populated with churches, monasteries, scriptural knowledge, increasing literacy, with a downturn in things such as human sacrifice and infanticide. Those who carry the burden of presenting Jesus Christ to those outside the faith are following in the steps of the apostles, sent out to make disciples of all nations. The apostles, of course, were for the most part fishermen, sent out to bring the "whole world into their net." It requires people like David, Brigit of Ireland, Columba, Gall, Francis Xavier, and modern day laborers in the "fields white unto harvest." There is no rest, each generation needing the Gospel taught and lived, to show that Christ is indeed with us even now in our midst, "till the end of the age." Were St. David alive today, what would he do? Just as he did before. What he do with a bank holiday? Preach!