Well, the earliest English king that's listed in my almanac is Egbert, 829 a.d., so Arthur has to have been considerably before that. I know we have scholars with the right references at their fingertips to pin it down better than that, but I'll leave them to it. My point is different.
The legendary (in whatever sense of the word) King Arthur was himself the product of earlier legends and tales, the truth value of which was largely unknown even then, let alone all these years since. The stories we have of King Arthur, therefore, are of a literally unknown truth value.
That's not what they are for, after all. They are to bring forward the underlying myths, legends and TRUTHS that are behind the stories and the decisions made over time of how to tell those stories. The symbolic value of the stories, if you will.
Arthurian legend is about chivalry, more than anything else. Indeed, it defined chivalry for a very long time. It's about the ways in which men protect women, and their families and their homes, and the respectful, dignified ways in which they went about this. It is a story of Kings and Queens and of noble activities; noble in the courtly sense as a metaphor and a symbol of the nobility of the heart. How does a gentleman treat a lady? Obviously, he loves her. He cares for her safety and well-being, and he loves to be in her company, and give her little treats and attentions to make her smile at him. He is, if you will, courtly.
And in life as in Arthurian legend, such behavior of gentleman toward lady is rewarded very, very richly. This is symbolized in the Quest for the Holy Grail which is a central theme of Arthurian legend. The symbolic value of that grail should be obvious enough to anyone with even a nodding acquaintance with the symbol system of the Tarot.