mar 22, 1187 - The affsir is known
With this event happening close to Arthur's death, this is an estimate point on when it occurred. Mordred hated Lancelot and found him and Guinevere having oddly time together. he called them out on it and a plan was made to catch them. A bunch of knights pilled up and caught Lancelot sneak into Guinevere's chamber. Lancelot ran away and Guinevere was sentenced to be burned.
Added to timeline:
King Arthur's lifetime