The first Christians felt something very theologically significant happened on a Sunday (“The First day of the week”).
This is relevant because absent some significant inciting event (like belief that Christianity's central event—Jesus' resurrection—took place on that day), nothing would've lead them to judge it so theologically significant.
The apostle Peter was publicly proclaiming that “Jesus resurrected!”
A full page will analyze these 7 arguments:
This is relevant because Peter was an early Christian, and moreover Christians at large followed him in his teachings/belief regarding Jesus (including that Jesus rose and appeared to him).