'First Day' in the Bible

On the first day [of the festival] of Unleavened Bread, when [as was customary] they sacrificed the Passover lamb, His disciples asked Him, “Where do You want us to go and prepare for You to eat the Passover?”

Very early on the first day of the week, they came to the tomb when the sun had risen.

[Now Jesus, having risen [from death] early on the first day of the week, appeared first to Mary Magdalene, from whom He had cast out seven demons.