We often hear about the twelve tribes of Israel, named for the twelve sons of Jacob: Reuben, Simeon, Levi, Judah, Dan, Naphtali, Gad, Asher, Issachar, Zebulun, Joseph, and Bejamin. They’re almost always referred to as the twelve tribes, however there are actually thirteen tribes. In Genesis 48, after Joseph is reunited with his brothers, Jacob adopts Joseph’s sons Ephraim and Manasseh as his own. There is no tribe named after Joseph; there are two tribes, Ephraim and Manasseh, named for his sons. The twelve tribes are actually thirteen.
Many assume the twelve tribes correspond to the twelve months of the year. In the Jewish calendar, the twelve months are: Nissan, Iyar, Sivan, Tammuz, Av, Elul, Tishrei, Cheshvan, Kislev, Tevet, Shevat, and Adar. However, unlike the Gregorian calendar that is used in the West, the Jewish calendar is based on the lunar month, so they don’t have as many days. For leap years, they don’t just add a day, they add an entire month. The month of Adar becomes the months Adar Alef and Adar Beit. The twelve months are actually thirteen.
When Jesus began His ministry, He chose twelve Apostles: Peter, Andrew, James son of Zebedee, John, Philip, Bartholomew, Thomas, Matthew, James son of Alphaeus, Judas son of James, Simon, and Judas Iscariot. However Judas (Judah) betrayed Jesus and subsequently hung himself. In Acts 1, Matthias is chosen to take his place. But then, in Acts 9, Jesus appears to a man who had been persecuting the followers of Jesus, a Pharisee named Saul, and he also becomes an Apostle. So the twelve Apostles are actually thirteen.
I’m not really sure what this means or what the significance is, but it is a very interesting pattern.