The Mystery Surrounding Jesus' Birth Date
Christians generally celebrate Jesus' birth on December 25, Christmas Day (some Orthodox Christians, however, celebrate Christmas on January 7, per Christianity). In reality, most historians do not think that Jesus was born that day, or, in fact, in A.D. 1, despite that this dating system is named for the birth of Jesus (B.C. stands for "before Christ" and A.D. stands for the Latin phrase "anno domini," meaning "In the year of the Lord," according to LiveScience).
Though historians have some evidence as to when Jesus died — the first Friday of April in either A.D. 30 or A.D. 33 (via the Center of Biblical Studies) — his date of birth is much less certain, simply because Jesus was not famous at the time of his birth like he was at the time of his death. Instead, his birthday was likely chosen by early Christians to coincide with Saturnalia, a Roman gift-giving festival celebrated near the winter solstice (per Britannica).
Was Jesus a September baby?
Today, historians and religious scholars believe that the historical Jesus was born in Bethlehem at some time between 6 and 4 B.C., since the biblical account of his birth is closely associated with the Judean King Herod, who died in 4 B.C. (via Got Question). This means that Jesus died in his mid-30s, which also fits with the description in the Gospel of Luke that Jesus began his brief ministry at about 30 years old. The Gospel of Luke, which includes the most thorough story of Jesus's birth, was likely composed around A.D. 63 to 70, and probably sourced from an earlier account, according to Britannica.
The exact day of Jesus's birth is a greater mystery. Some Biblical scholars believe Jesus was likely born in the summer or fall, since the Gospel of Luke mentions that shepherds were watching their flocks in fields at the time (by winter, shepherds would have moved their flocks inside, per Christianity). According to Luke, Mary was visited by an angel in the sixth month of her relative Elizabeth's pregnancy with John the Baptist. Since John was conceived in June, that places Jesus's birth in September.