'First Day' in the Bible

[In the celebration of the Passover in future years,] seven days you shall eat unleavened bread, but on the first day you shall remove the leaven from your houses [because it represents the spread of sin]; for whoever eats leavened bread on the first day through the seventh day, that person shall be cut off and excluded from [the atonement made for] Israel.

On the first day [of the feast] you shall have a holy and solemn assembly, and on the seventh day there shall be another holy and solemn assembly; no work of any kind shall be done on those days, except for the preparation of food which every person must eat—only that may be done by you.

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

“On the first day of the first month (Abib) you shall set up the tabernacle of the Tent of Meeting [of God with you].

Now it happened on the first day of the first month (Abib) in the second year [after the exodus from Egypt], that the tabernacle was erected.