Knight for a Day is set in 533 AD Britain, during what used to be known as The Dark Ages but is now called the sub-Roman period. This was the most likely time for King Arthur to have reigned if he truly existed, a century after the last Roman legions left in 410 and while Christian Celts and the descendants of the Romans were still holding out in the west against the Germanic Anglo-Saxons.
The battle of Mount Badon (c.500) was reputed to be the last significant Romano-Celtic victory against these invaders. It has been linked to King Arthur, as mentioned in Knight for a Day, but no details are known about it.
In 5th century Europe six major Germanic tribes, the Franks, Vandals, Vizigoths, Ostrogoths, Burgundians and Lombards triggered the collapse of the Western Roman Empire and replaced it with their own kingdoms. However the early 530's saw many of these being broken up or conquered. It was a time of increasingly bitter schisms and religious conflicts between Catholics and Arians: Christians who rejected the Pope's authority, the Holy Trinity and Christ's divinity.
In Gaul King Clovis 1 had united all the Frankish tribes by the year 486 and was the first king of what would become France - Clovis is the origin of the name Louis, borne by 18 later kings - and he became a Catholic in 496, which led to a widespread conversion of the Frankish peoples. Clovis died in c.511 and his realm was divided between his four sons, creating the kingdoms of Rheims, Orleans, Soissons and Paris. By 533 their rivalry had turned to gruesome family murders and bloodshed.
Further south the former imperial capital of Rome had been overrun and sacked in 455 by the Vandals, who were in turn driven out by the Arian Ostrogoths from the Black Sea area. In 533 the Ostrogoths had no effective ruler following the death of Theodoric the Great in 526, leaving an infant grandson as his heir and creating a power vacuum.
The Vandals had already lost southern Spain (Vandal is the root of the name Andalucia) to the Arian Visigoths by 530, when the Catholic Vandal king of North Africa was deposed by his Arian cousin, Gelimer. This gave the Catholic Eastern Roman Byzantine Emperor, Justinian 1, a convenient pretext to intervene.
In his capital of Constantinople, Justinian had only narrowly survived the Nika riots of 532 by massacring 30,000 of his rebellious citizens in the hippodrome, and was rebuilding Hagia Sophia, the church of the holy wisdom, as the largest and most splendid building the world had ever seen. In 533 he sent a small army under his brilliant general Belisarius across the Mediterranean to try to win back North Africa, and in two stunning victories he shattered the divided Vandal kingdom in a few months. On his return Belisarius was awarded the last Roman military triumph in history, and a couple of years later he also briefly regained control of Rome and Italy from the Ostrogoths.