'First Day' in the Bible

Seven days you shall eat unleavened bread, but on the first day you shall remove leaven from your houses; for whoever eats anything leavened from the first day until the seventh day, that person shall be cut off from Israel.

On the first day you shall have a holy assembly, and another holy assembly on the seventh day; no work at all shall be done on them, except what must be eaten by every person, that alone may be prepared 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.

“On the first day of the first month you shall set up the tabernacle of the tent of meeting.

Now in the first month of the second year, on the first day of the month, the tabernacle was erected.