Life's Little Mysteries

Why is Christmas celebrated on Dec. 25?

A father and daughter decorate a tree in anticipation of Christmas. But why is Christmas celebrated on Dec. 25? (Image credit: 10'000 Hours via Getty Images)

Today, many (but not all) Christians celebrate Christmas on Dec. 25. But given that the birth date of Jesus Christ is not known with certainty, why was this day selected? 

There are two major theories as to why Christmas is celebrated on Dec. 25. One, sometimes called the "history of religion" hypothesis, suggests that Christmas supplanted one or more pagan holidays. The other theory, often called the "computation" or "calculation" hypothesis, suggests that early Christians used some form of calculation to arrive at Dec. 25 as Jesus' birthday. 

In fact, both theories may be true. "The two hypotheses are not mutually exclusive," Philipp Nothaft, a fellow at the University of Oxford's All Souls College, whose research looks at the history of astronomy and chronology, told Live Science in an email. Nothaft has researched and written about the date of Christmas. 

Related: Who is Krampus, and what does he have to do with Christmas?

Two theories

It's unclear exactly when and why some Christians started celebrating Jesus' birth on Dec. 25. 

Ancient records indicate that a feast dedicated to Sol Invictus, a sun god, was held in the Roman Empire on Dec. 25, raising the possibility that Christmas replaced it. There was also a pagan festival called Saturnalia in mid-December that occurred over several days.

However, there are some problems with this so-called history-of-religion theory: Christians may have been celebrating Jesus' birthday on Dec. 25 before the Sol Invictus feast was created, Paul Bradshaw, a professor emeritus of theology at the University of Notre Dame, wrote in an article published in the book "The Oxford Handbook of Christmas" (Oxford University Press, 2020). 

Nothaft agreed. "A lot hinges on when 25 December became the occasion of a Roman feast associated with [Sol Invictus]," he said in the email. "Most scholars would probably agree that this is unlikely to be earlier than AD 274, the year when [Emperor] Aurelian dedicated a new temple to Sol Invictus in Rome.

"We know too little about this feast to make confident pronouncements," Nothaft added. There is also the question of whether the feast was significant enough for early Christians to place Jesus' birth on that day.

Dec 25 is also "the date on which the Northern Hemisphere observers are first able to detect the northward movement of the sun" after the winter solstice, David Allen, who was an astronomer at the Anglo-Australian Observatory, wrote in a 1992 article published in the journal Archaeoastronomy. The detection of this movement may explain why the festival was held on this date, Allen noted.

The other theory, the computation hypothesis, is based on the idea that early Christians calculated Jesus' birthday by adding nine months to a day they regarded as Jesus' conception. One possibility is that some early Christians believed that the day of Jesus' crucifixion occurred on March 25 and they added nine months to come up with Dec. 25. This means that early Christians considered the date of Jesus's crucifixion to be the date of conception, noted Bradshaw. 

One piece of evidence for this hypothesis is a third-century inscription on a statue that gives calculations for when Easter should be celebrated and has an inscription on it noting that Jesus was crucified on March 25 in the year 29, Bradshaw wrote in his article. (Scholars tend to think that Jesus, who died at age 33, was born around 4 B.C.)

A problem with the computation hypothesis is that it's unclear why early Christians would associate March 25 as the date of both Jesus' crucifixion and conception, Bradshaw wrote. 

"There is uncertainty around both theories, but I do think the computation hypothesis has a slight edge," Bradshaw told Live Science in an email.

It is possible that both theories are correct, Nothaft said. "I am inclined to accept that the tradition of dating Christ's birth to 25 December has roots that go back at least to the beginning of the 3rd century," he said. "If this is so, some version of the calculation hypothesis, together with Christian solar symbolism, offers the best explanation as to why this tradition originated."

Editor's note: Updated at 12:05 p.m. EST on Dec. 12 to correct Philipp Nothaft's name.

Owen Jarus
Live Science Contributor

Owen Jarus is a regular contributor to Live Science who writes about archaeology and humans' past. He has also written for The Independent (UK), The Canadian Press (CP) and The Associated Press (AP), among others. Owen has a bachelor of arts degree from the University of Toronto and a journalism degree from Ryerson University. 

  • Big Crow
    Jesus was 33 years old when crucified, always wondered about that. At 66 it makes me feel lucky to be alive.
  • Hildy
    The biggest argument against calculation is that the knowledge at the time derived from Hippocrates who declared that pregnancy lasted 8 months.

    The biggest argument for the adaptation of a pre-Roman pagan tradition is the existence of the shepherds who watched their flocks by night. This is indeed something shepherds do but the do it in the spring when the ewes are giving birth so they can assist with difficult births, not during the Christmas season.
  • Archangel Raphael
    If Dec. 25th was derived from pagan influence ... then the birth of Jesus associated to Dec. 25th is clearly a reference to the sun 'standing still' during the winter solstice. The winter solstice is when the days starting getting longer ... what is clearly not a coincidence is that Dec. 25th is associated to the birth of Jesus ... the 'sun' of god being reborn. Sol Invictus. Fast forward to the spring equinox which is aligned with Easter and when 'Jesus is crucified' both seem to be associated with the moon and a balance achieved (equinox) the length of the day equals the night.

    Btw what is Ostara? The answer is how the spring equinox became Easter.If as the article suggests the date of death of Jesus was March 25th .... then ask yourself why does the date of Jesus' death move around from one year to the next but his birth on Dec. 25th does not? Is there another factor involved?
    Is it because it is somewhat dependent on when the Jewish Passover occurs which is calculated using the cycles of the moon?
    Yes. is the simple answer. The Jewish Hebrew calendar is a lunar-solar whereas the Christian Gregorian calendar is solar.
    What is interesting about March 25 or 3/25 is that it was the Council of Nicea which convened in 325 AD that dealt with the controversies of when the month of Nissan actually began. The Council had separated the Easter computation from all dependence, positive or negative, on the Jewish calendar.
    Hence the life and death of Jesus is clearly entangled with a SUN + MOON narrative which is found in many CULTures around the world from which Christianity obviously borrowed.

    BTW Saturnalia was originally celebrated on December 17 before it was extended to three days and eventually to seven days. (hmm both 3 and 7 are considered mystical/magical numbers in Christianity)
    First 3 days, then 7 days, now compare to the 12 days of Christmas which last from Dec. 24/25 to Jan. 6/7 marking the Eastern Orthodox celebration Feast of the Epiphany.
    Is it a coincidence that 3, 7, 12 in fact are the 22 letters of the Jewish alphabet broken down into 3 mothers, 7 doubles and 12 simple letters?

    Saturnalia was a celebration in honor of Saturn. Hence we have two celebrations ... Saturn and the Sun.
    Which long ago were the two celestial objects that depicted the boundaries of our solar system using 'naked eye' observations from the Sun out to Saturn.

    What was Saturnalia?
    Saturnalia, the most popular holiday on the ancient Roman calendar, derived from older farming-related rituals of midwinter and the winter solstice, especially the practice of offering gifts or sacrifices to the gods during the winter sowing season. (Hence the scythe associated with Saturn)
    People decorated their homes with wreaths and other greenery, and shed their traditional togas in favor of colorful clothes known as synthesis. Even enslaved people did not have to work during Saturnalia, but were allowed to participate in the festivities; in some cases, they sat at the head of the table while their masters served them.