Step Day was the day in 2015 when Willis Linsay uploaded the circuit diagram for the Stepper Box onto the Internet.[1][2]
The Stepper Box caused the Long Earth to become a phenomenon known by all humans, and made stepping possible for everyone (rather than just natural steppers, like in the past).[3]
The Long Earth[]
On Step Day, when the diagram was first released, kids from all around the world started putting together Steppers and disappeared.[2]