'First Day' in the Bible

On the first day of the Festival of the Unleavened bread, when it was customary to kill the Passover lambs, his disciples said to Jesus: "Where do you wish us to go and make preparations for your eating the Passover?"

And very early on the first day of the week they went to the tomb, after sunrise.

After his rising again, early on the first day of the week, Jesus appeared first of all to Mary of Magdala, from whom he had driven out seven demons.