Philip Jenkins on why we are certain March 1 was the day the patron saint of Wales died and why "death days" were so important:
...[W]e really know very little about David that is historically solid and can only guess at his dates, or his main areas of activity. A death about 590 is a reasonable guess, but we could easily slip fifty years either way. Oddly though, we can be sure that he died on March 1, whether in (say) 532 or 632 AD. Through the Middle Ages, hagiography was a vast area of cultural effort, when almost any outrageous achievements could be credited to a saint. (No, David did not really make a pilgrimage to Jerusalem, where he was ordained by the Patriarch). The one thing that we know these writers did keep faithfully was the death day – the date not the year – because that marked the hero’s ascension to glory, the promotion to heaven. In a particular church or community, those days were critical, as marking the annual celebration of the beloved local saint.
St David's, Pembrokeshire, Wales
Argue as much as you like, then, about precise years, achievements, martyrdoms and areas of activity, about the number of lepers cured and tyrants opposed – but don’t quarrel with death days.
It’s an interesting term. I know my birthday. I also know that at some future point I will die, and that that will befall on a particular date. Let me be optimistic and assume that it will be a distant event, say on July 23, 2049. Each year, then, I pass through July 23 happily unaware that I am marking my Death Day, surely as significant a milestone as my birthday, but not one I can ever know with certainty until it occurs. Nor is it something we really ever contemplate, as we all know, in our hearts, that we are immortal.
I suppose though that it is something we can learn from those medieval monks, that the Death Day is not just a key event in anyone’s life, but literally the only one we can take with absolute confidence. [emphasis added]