'Fifteenth' in the Bible

And in the fifteenth day of this month is the feast: seven days shall unleavened bread be eaten.

And on the fifteenth day of the seventh month ye shall have an holy convocation; ye shall do no servile work, and ye shall keep a feast unto the LORD seven days:

And they departed from Rameses in the first month, on the fifteenth day of the first month; on the morrow after the passover the children of Israel went out with an high hand in the sight of all the Egyptians.