Otherwise known, after Bede, as the Adventus Saxonum.
Gildas in the De Excidio Britanniae describes the circumstances in which the Anglo-Saxons first arrived in Post-Roman Britain,
Then all the councillors, together with that proud tyrant, the British king, ... (invited) ... the fierce and impious Saxons to repel the invasions of the northern nations
The proud tyrant was Vortigern, and there is archaeological evidence of Anglo-Saxon settlements in Kent and around Londinium at about the mid 5th century. Hence we cam readily accept that these Saxon mercenaries did arrive, but the question remains as to when they arrived as Gildas neglects to offer us any clues.
Traditionally historians have looked to Bede for the answer, who states that,
In the year of our Lord 449 Marcian, forty-sixth from Augustus, became emperor with Valentinian and ruled for seven years. At that time the race of the Angles or Saxons, invited by Vortigern, came to Britain
On the other hand the Historia Brittonum states that,
Vortigern reigned in Britain when Theodosius and Valentinian were consuls, and in the fourth year of his reign the Saxons came to Britain, in the consulship of Felix and Taurus, in the four hundredth year from the Incarnation of our Lord Jesus Christ.
Which, I understand, means that Vortigern came to power in 425 AD, and the Saxons arrived in 429 AD.
Which leaves us with two different dates, twenty years apart. And if that was not bad enough there is the evidence of the Gallic Chronicles that state that in the year 441 AD Britain was "reduced to Saxon rule".
Clearly if the Anglo-Saxons were running amok in Britain in 441 AD then they arrived before 449 AD.
A way out of this impasse may be suggested in the following manner.
The adventus that the Historia Brittonum speaks of is the first arrival of the mercenaries in 429 AD. Some time before 441 AD these mercenaries revolt, and perhaps togther with fresh incursions of Saxon invaders take control of a large portion of Britain.
The adventus that Bede speaks of is the point in time at which these invaders established sufficient control within the ebb and flow of the following war to establish their own kingdom within Britain. (And
Bede of course, spoke only of the oral tradition of the founding of the kingdom of Kent and was making his own best-estimate of the date.)
The difference in dating may therefore be due entirely to the different perspectives of the various histories.
Part of the Sub-Roman Britain project, where sources are detailed.