In the Bible, God set dates and feasts based on the changes of the moon, dividing the year into 12 months to represent the 12 tribes of Israel. According to the Jewish calendar, which was created from the changes of the moon, the moon's orbital period is 29.5 days. Therefore, on average, there are seven leap months in a 19-year period. The Earth goes through the change of seasons once a year, orbiting the sun for 365 days before returning to its original position. So if you calculate the Earth's rotation period of 365 days and the moon's rotation period, there is a difference of about 11 days per year. So to eliminate this difference, the Jewish calendar inserts 7 leap months every 19 years. So if you look at the Jewish calendar with leap months, it looks like this:

January - Nissan

February - Iyar

March - Sivan

April - Tammuz

May - Av

June - Elul

July - Tishrei

August - Cheshvan

September - Kislev

October - Tevet

November - Shevat

December - Adar I

December – Adal 2 (13th month) 

In the Jewish calendar, a leap month is added after the month of Adar (December), so a month with a leap month adds an extra month to the 13th month, making the year 13 months long instead of 12. But this tells us the secret of Jacob's 12 sons. Jacob had 12 sons, and there's a secret in that. In the Bible, Jacob's eleventh son, Joseph, was hated by his brothers and sold into the land of Egypt. However, he became prime minister in Egypt and had two sons, Ephraim and Manasseh. After Joseph was gone, Jacob's father, Jacob, had a son named Benjamin by his wife Rachel. This gave rise to Jacob's twelve sons, who were named as follows. 

First - Reuben

Second - Simeon

Third - Levi

Fourth - Judas

Fifth - Dan

Sixth - Naphtali

Seventh - Gad

Eighth - Asher

Ninth - Issachar

Tenth - Zebulun

Eleventh - Joseph

Twelfth - Benjamin 

However, a famine forces Jacob and his family to migrate from the land of Canaan to Egypt, and shortly before he dies, Jacob adopts Joseph's two sons, Ephraim and Manasseh. 

[Genesis 48:5~6] "Now then, your two sons born to you in Egypt before I came to you here will be reckoned as mine; Ephraim and Manasseh will be mine, just as Reuben and Simeon are mine. Any children born to you after them will be yours; in the territory they inherit they will be reckoned under the names of their brothers. 

In this way, Jacob adopted Joseph's two sons, Ephraim and Manasseh. As a result, Israel was divided into 13 tribes, and the tribe of Joseph was named after his eldest son, Manasseh. Therefore, the added tribe was Ephraim, and Joseph received the same name as Manasseh. So, what prophecy is hidden between Joseph, who grew up in Egypt, and his two sons? Yeshua said this about it: 

[John 10:16] And other sheep I have, which are not of this fold: them also I must bring, and they shall hear my voice; and there shall be one fold, and one shepherd. 

Yehoshua said that there are other sheep that do not belong to this fold. It refers to Joseph and his two sons, who grew up in Egypt. Jacob adopted Joseph's two sons. And he divided the portion of the tribe among his two sons. Therefore, the other sheep that are not of the fold are the two tribes of Joseph and his two sons. When God instituted the Jewish calendar, He divided the year into 12 months, but when leap months are included, it becomes 13 months. However, this was done to reveal the twelve tribes of Israel and the two sons of Joseph. Therefore, if you study it carefully, you will discover the secret of the remaining tribes to be saved. The two tribes of Joseph will be saved when the work of the two witnesses is revealed.