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!”
See this full article to analyzed 4 evidences:
• Paul says this about Peter (#) Acts says this about Peter Peter belived Jesus visited him after dying
• All the apostles said Jesus rose
• The Jerusalem church said Jesus rose
• 1 Cor 15:1-11 says Jesus rose
• 1 Peter 1 & 3 says Jesus rose • Peter was martyred