Alban, Protomartyr of Britain was the first Christian martyr in Britain. The first mention of St. Alban is by Constantius, in his Life of St Germanus of Auxerre, written about 480. His feast day is June 22.
According to Bede's Ecclesiastical History, I.vii and xviii, Alban was a pagan living at Verulamium (modern day St Albans, England}, who converted to Christianity, and was executed by beheading on a hill above the Roman settlement of Verulamium. St. Albans Abbey at St Albans, Hertfordshire, England was later founded near this site.
The date of the execution is best left to the Venerable Bede: "when the cruel Emperors first published their edicts against the Christians" in other words, sometime prior to Christianity becoming an officially tolerated religion of the Roman Empire under Emperor Constantine in 324, when local Christians were persecuted by the Romans.
Alban sheltered a Christian priest, (Geoffrey of Monmouth's later interpolation giving his name as "Amphibalus," the name for a cloak) in his home, and was converted and baptised by him. When the "impious prince" (as Bede has it) sent Roman soldiers to Alban's house to look for the priest, Alban exchanged cloaks with the priest and was arrested in his stead. Alban was taken before the magistrate, where he avowed his new Christian faith and was condemned for it. He was beheaded on the spot where St. Alban's Cathedral now stands.
A cult connected with Alban was already in existence in the 6th century, for Bede quotes a line from one of the Carmina of Venantius Fortunatus, Albanum egregium fæcunda Britannia profert ("Fruitful Britain holy Alban yields").
Bede tells several legends associated with the story of Alban's execution. On his way to the execution, Alban had to cross a river, and finding the bridge full of people, he made the waters part and crossed over on dry land. And the executioner was so impressed with Alban's faith that he also converted to Christianity on the spot, and refused to kill him. Another executioner was quickly found (whose eyes dropped out of his head when he did the deed), and the first was killed after Alban, becoming the second British martyr for Christ.
Some details added to St. Alban's tradition come from confusing him with another St. Alban, or Albinus, who was martyred at Mainz.