'First Day' in the Bible

Seven days shall ye eat unleavened bread; even the first day ye shall put away leaven out of your houses: for whosoever eateth leavened bread from the first day until the seventh day, that soul shall be cut off from Israel.

And in the first day there shall be to you a holy convocation, and in the seventh day a holy convocation; no manner of work shall be done in them, save that which every man must eat, that only may be done by you.

On the first day of the first month shalt thou rear up the tabernacle of the tent of meeting.

And it came to pass in the first month in the second year, on the first day of the month, that the tabernacle was reared up.