'Fifteenth' in the Bible

There shall be a feast on the fifteenth day of this month; unleavened bread shall be eaten for seven days.

‘Then on the fifteenth day of the seventh month you shall have a holy [summoned] assembly; you shall do no laborious work, and you shall observe a Feast [of Booths] to the Lord for seven days.

They set out from Rameses on the fifteenth day of the first month; on the day after the Passover the Israelites moved out triumphantly in the sight of all the Egyptians,