'First Day' in the Bible

You are to eat unleavened bread for seven days. On the first day be sure to remove all the leaven from your houses, because any person who eats anything leavened from the first day until the seventh will be cut off from Israel.

Also, on the first day you're to hold a holy assembly, and on the seventh day you're to hold a holy assembly. No work is to be done during those days, except for preparing what is to be eaten by each person.

In the first month, from the evening of the fourteenth day of the month until the evening of the twenty-first day of the month, you are to eat unleavened bread.

"On the first day of the first month you are to set up the tent of the Tent of Meeting.

And so in the first month of the second year, on the first day of the month, the tent was set up.