No founder, establishment record, proclamation or launch announcement for National Stephen Day has been identified. The day circulates on holiday listings and has been marked by at least one newsroom, but nothing documents who began it or when.
From cloisters to the throne
In England the name stayed narrow for centuries. Etymonline puts it plainly: it was exclusively a monk's name in Anglo-Saxon England, and it became popular after the Conquest. Behind the Name reaches the same conclusion independently, crediting the Normans with popularizing it.
Then it went to the top. Britannica lists Stephen of Blois, a grandson of William the Conqueror, as king of England from 1135. He held the crown until his death, and the civil war with his cousin Matilda swallowed most of the reign.


