This is the feast date and martyrdom date of middle ages English king Edmund the Martyr.
This acute ruler of the East Angles, the last native East Anglian king, was stomped in battle by the marauding norsemen under Ivar the Boneless and his less interestingly-named brother Ubbe Ragnarsson.
These two were sore about their father Ragnar Lodbrok, who had shipwrecked in England — maybe East Anglia, maybe elsewhere — and allegedly been thrown into a snakepit.
According to the hagiographic account, these Danish heathens attempted to force Edmund to renounce Christianity. Edmund demurred.
Then those wicked men bound Edmund, and shamefully insulted him, and beat him with clubs, and afterwards they led the faithful king to an earth-fast tree, and tied him thereto with hard bonds, and afterwards scourged him a long while with whips, and ever he called, between the blows, with true faith, on Jesus Christ; and then the heathen because of his faith were madly angry, because he called upon Christ to help him. They shot at him with javelins as if for their amusement until he was all beset with their shots, as with a porcupine’s bristles, even as Sebastian was.
The martyr-king’s body was ultimately interred at the aptly-namd Bury St. Edmunds. This locale thereafter became a major, and lucrative, pilgrimage spot in Britain.
Edmund himself became the patron saint of England until he was supplanted just before the Norman invasion by omnibus patron saint George. As George had nothing to do with England, there’s been some latter-day push to revert the honor to the native king.
So far, no dice.
Update: Jamie that killjoy at the British History Podcast puts it in 869 and rebuts the notion that there was any execution at all, here.