The number 15 in the Bible pictures rest, which comes after deliverance, represented by fourteen. The 15th day of the first Hebrew month (Nisan) is the first day of the Feast of Unleavened Bread, a day of rest for the children of Israel (and for Christians). The 15th day of the 7th Hebrew month begins the Feast of Tabernacles, also a day of rest.
God told Abraham in a vision, just as the sun was setting to begin Nisan 15 on the Hebrew calendar, that his descendants would end up as slaves in a foreign country (Egypt). They would, however, eventually be set free (Genesis 15:12 - 16).
Many years later in Egypt, God miraculously delivered Israel's firstborn from the death angel just as Passover began after sunset (Nisan 14). Then, 24 hours later (just as the sun was setting to begin Nisan 15), the children of Israel began to leave Egypt (Exodus 12:40 - 41). This night is referred to as the 'night to be much observed' (Exodus 12:40 - 42, Deuteronomy 16:1). God's prophecy of freedom, given to Abraham on Nisan 15, was fulfilled years later on the exact same day.