'First Day' in the Bible

You shall eat unleavened bread seven days; even the first day you shall put away leaven out of your houses. For whoever eats leavened bread from the first day until the seventh day, that soul shall be cut off from Israel.

And on the first day shall be a holy gathering, and in the seventh day there shall be a holy gathering for you. No manner of work shall be done in them, except that which every man must eat, that only may be done by you.

In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty-first day of the month at evening.

You shall set up the tabernacle of the tent of the congregation on the first day of the first month.