Northumbria Day is August 31st, the feast day of Saint Aidan.
Aidan of Lindisfarne was an Irish monk and missionary credited with restoring Christianity to Northumbria. He founded a monastic cathedral on the island of Lindisfarne, served as its first bishop, and travelled ceaselessly throughout the countryside, spreading the gospel to both the Anglo-Saxon nobility and to the socially disenfranchised, including children and slaves. He is known as the Apostle of Northumbria and is recognised as a saint by the Orthodox Church, the Roman Catholic Church, the Anglican Communion and others. His date of death is recorded as August 31st, also recognised as his feast day and subsequently as Northumbria Day.