Why is Christmas observed on December 25th?
Christians believe that Jesus Christ is the Son of god, and christmas is a time to commemorate his birth. Nobody is aware of Jesus' actual birthdate! Since the bible doesn't provide a date, why do we celebrate christmas on december 25th? When to celebrate it was a topic of much debate among the early Christians! Furthermore, Jesus' birth most likely occurred a little earlier—between 2 and 7 BCE/BC, perhaps even in 4 BCE/BC (there isn't a 0—the years go from 1 BC/BCE to 1!).
The first "official" record of christmas being observed on december 25th dates back to 336, when Constantine, the first Christian Roman Emperor, ruled. However, at this time, it was not a recognized Roman state celebration. However, the reasons why christmas is observed on december 25th are the subject of numerous customs and hypotheses.
According to a very old Christian belief, the Annunciation—the day Mary was informed that she would give birth to Jesus—took place on march 25. This date is still commemorated on march 25. The 25th of december comes nine months after the 25th of March!
Some early Christians also believed that the world was created on march 25th, and that Jesus died on that day as an adult. They believed that Jesus was conceived and died on the same day of the year. According to the Jewish calendar, Jesus died on Nisan 14, which is also the day of the Jewish holiday of Passover. Unlike 'fixed' calendars like the Gregorian (or earlier Julian) calendar, the Jewish calendar is lunar (based on the moon, not definite dates), hence it fluctuates.
An early church historian named Sextus Julius Africanus (c. 160–c. 240) is cited in the year 221 as the primary source for the dating of Jesus' conception on march 25. Nonetheless, a large portion of Sextus Julius Africanus's writings and quotations are taken from later sources. According to the teachings of Saint Ephrem the Syrian (306–373), Jesus was conceived on Nisan 10! In order to designate these "moveable" dates on the Jewish calendar, march 25th was made a "fixed" date.