'First Day' in the Bible

Seven days, unleavened cakes, shall ye eat, surely, on the first day, shall ye put away leaven, out of your houses, - for whosoever eateth what is leavened, then shall that soul be cut off out of Israel, from the first day, unto the seventh day.

Both on the first day, a holy convocation, and on the seventh day - a holy convocation, shall there be to you, - no work, shall be done therein, save only what must be eaten by every soul, that alone, shall be done by you.

On the first day of the month, in the first month, shalt thou rear the habitation, of the tent of meeting;