'First Day' in the Bible

Now on the first day of the Festival of Unleavened Bread [i.e., the beginning of the Passover Festival week], when they sacrificed the Passover [lamb], Jesus' disciples said to Him, "Where do you want us to go to make preparations for you to eat the Passover meal?"

And very early on the first day of the week, after sunrise [i.e., on Sunday morning], they went to the grave site.

Now when Jesus arose from the dead early on the first day of the week [i.e., Sunday morning], He appeared first to Mary from Magdala, from whom He had driven out seven evil spirits.