Lit. 'the advent or arrival of the *Saxons (in England)'. Just when they began to settle here is far from clear. Between the departure of the Romans in 410 and the late 6c there is not much that can with certainty be given a year date. Bede has the Saxons settling 450-5; the *ASC gives 449 as their first entry into England.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.