Constantine the Great was born on 27 February — year unknown

Constantine the Great, the first Roman emperor to profess Christianity, was born on 27 February at Naissus (modern Niš, Serbia), in the year 271, 272 or 273 — the day is well attested, but the year is not. His father, Constantius, was an army officer who rose to emperor; his mother, Helena, was a woman of humble background later venerated as a saint.

How we know the day but not the year

The day survives because Rome celebrated it. The Calendar of 354, an illustrated almanac compiled in Rome, marks 27 February as N(atalis) D(ivi) Constantini — the birthday of the deified Constantine — with 24 chariot races, still on the books seventeen years after his death. The same calendar gives the same 24 races on 25 July, the day in 306 when soldiers at York proclaimed him emperor — his dies imperii, or imperial birthday. The year of his birth is far less secure, which is why scholars still offer three candidates.

His Christian record is also often simplified. Constantine did end persecution in his own territories when he came to power. But two years before Milan, the senior emperor Galerius had already issued an edict, in 311, conceding that Christians “may again be Christians” and hold their meetings. The so-called Edict of Milan of 313 is misnamed: it was issued by his co-emperor Licinius, after the two had agreed a common religious policy at Milan.

Roman hostility to the faith reached back to Nero, who blamed Christians for the Great Fire of Rome in AD 64.

Constantine was baptised only shortly before he died at Nicomedia on 22 May 337 — not a sign of insincerity, since many fourth-century Christians delayed baptism, but notably by an Arian bishop, Eusebius of Nicomedia. By then he had refounded Byzantium as Constantinople, dedicated on 11 May 330, which outlived the western empire by almost a thousand years.