'First Day' in the Bible

They supposed that he was somewhere in the party, and traveled until the end of the first day's journey, and then they looked everywhere for him among their relatives and acquaintances.

Then came the first day of Unleavened Bread on which the Passover lamb had to be sacrificed.

But on the first day of the week, at early dawn, they came unto the tomb, bringing the spices which they had prepared.