'Fifteenth' in the Bible

On the fifteenth day of this month there is to be a feast; for seven days let your food be unleavened cakes.

And on the fifteenth day of the seventh month let there be a holy meeting; do no field-work, and keep a feast to the Lord for seven days;

On the fifteenth day of the first month they went out from Rameses; on the day after the Passover the children of Israel went out by the power of the Lord before the eyes of all the Egyptians,