What is the Spiritual meaning of the number "fifteen" seeing that it occurs at significant portions of Scripture?

Question

1) The waters prevailed 15 cubits above all the mountains of the earth. (Gen 7:20)

2) The waters prevailed over the earth 150 days. For the ark to later come to rest on the mountains of Ararat, the waters had to recede 15 cubits in 150 days, 1 cubit every 10 days. (Gen 7:24)

3) The hangings for the gate of the court in the tabernacle were 15 cubits on both sides. (Exo 27:14-15)

4) The feast of unleavened bread begins on the 15th day of the first month. (Lev 23:6)

5) Israel left Rameses on the 15th day of the first month. (Num 33:3)

6) God added 15 years to the life of Hezekiah. (II Kings 20:6)

7) Esther and Mordecai established the feasts of Purim on the 14th & 15th day of Adar. (Esther 9:18)

8) Luke 3:1 begins the story of John the baptist by stating that he began his ministry in the 15th year of Tiberius Caesar.

9) Bethany was 15 stadia from Jerusalem. (John 11:18)

10) After sounding 15 fathoms, the sailors on Paul's ship started praying for day light. (Acts 27:28)

11) Paul stayed 15 days with Peter. (Gal 1:18)

So what is the Spiritual meaning of the number FIFTEEN as it occurs and is used in Scripture? And what are the rules used in determining numeric symbolisms?

Answer 1

From "Meaning of the Numbers in the Bible" web page:

"The Meaning of Numbers: The Number 15

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.

After dying on the cross, Jesus' body is placed in a Garden Tomb as the sun was setting to begin Nisan 15 in 30 A.D. (Matthew 27:57 - 61, Luke 23:50 - 55, John 19:38 - 42).

Appearances of the number fifteen

King Hezekiah, who was sick and dying, prayed that God would remember his works and how he served him with his whole heart. God heard his prayer and told the king, through the prophet Isaiah, that he would be healed and granted an additional number of years of live (15 - see 2Kings 20:1 - 6)."

Source: here

Answer 2

The significance of the fourteenth or fifteenth day of a month is the occurrence of a full moon on that date. The month starts with the new moon. It is another way of saying the night/day of the full moon.