Christmas is celebrated on December 25. But why on December 25?
First thing to point out here is that there is nothing on the bible that states December 25 is Christmas Day.
Early Christians agreed that there should be a certain day, a unified day, that all people celebrate the Birth of Christ. And December 25 is the date they all agreed upon.
But why December 25? They didn’t decide randomly. There’s a basis to it.

Zechariah (from Wikipedia)
ONCE UPON A TIME
Once there was a righteous man named Zacharias. He and his wife Elizabeth are old. They had no child because Elizabeth was barren.
A large number of people gathered outside the temple to pray. But it was Zacharias’ turn to burn incense, so he has to go inside the temple.
And while inside the temple, the angel of the Lord appeared to him. The angel told Zacharias that his wife shall bear a son.
The event that Zacharias attended was a Jewish Festival celebrated on the 18th of September until 24th of September.
After the said event, Zacharias’ wife began to concieve. (25th of September.)
Six months later, an angel appeared to Mary, the spouse of Joseph, announcing that she too will concieve a Child. His name is Jesus. He shall be great, and shall be called the Son of the Highest. And the Lord God shall give unto Him the throne of His father David. And He shall reign over the house of Jacob forever, and His kingdom there shall be no end.
So six months from September 25 is March 25. March 25 is celebrated nowadys as the feast of “The Annunciation”.
From March 25, Mary gave birth to the King of Kings after nine months of conception. And so nine months after March 25 is December 25.
And the world rejoices on that very day.