The basis for this key date is in the Anglo Saxon Chronicle - translated as follows:
This year appeared fiery lights in the northern part of the firmament; and Sihtric departed; and King Athelstan took to the kingdom of Northumbria, and governed all the kings that were in this island: -- First, Howel, King of West-Wales; and Constantine, King of the Scots; and Owen, King of Monmouth; and Aldred, the son of Eadulf, of Bamburgh. And with covenants and oaths they ratified their agreement in the place called Emmet, on the fourth day before the ides of July; and renounced all idolatry, and afterwards returned in peace
The Ides of July would be 15 July - hence four days before being 12 July. The word Ides comes from the Latin word "idus", a word that was used widely in the Roman calendar indicating the approximate day that was the middle of the month. The term ides was used for the 15th day of the months of March, May, July, and October, and the 13th day of the other months
This year appeared fiery lights in the northern part of the firmament; and Sihtric departed; and King Athelstan took to the kingdom of Northumbria, and governed all the kings that were in this island: -- First, Howel, King of West-Wales; and Constantine, King of the Scots; and Owen, King of Monmouth; and Aldred, the son of Eadulf, of Bamburgh. And with covenants and oaths they ratified their agreement in the place called Emmet, on the fourth day before the ides of July; and renounced all idolatry, and afterwards returned in peace
The Ides of July would be 15 July - hence four days before being 12 July. The word Ides comes from the Latin word "idus", a word that was used widely in the Roman calendar indicating the approximate day that was the middle of the month. The term ides was used for the 15th day of the months of March, May, July, and October, and the 13th day of the other months