It is generally believed that the birth of Christ is celebrated on December 25 because our savvy Christian forebears with a flare for marketing took over a winter solstice holiday from the surrounding pagans. Not so, apparently. Here is William J. Tighe on Calculating Christmas and Andrew McGowan on How December 25 Became Christmas. McGowan writes:
There is another way to account for the origins of Christmas on December 25: Strange as it may seem, the key to dating Jesus’ birth may lie in the dating of Jesus’ death at Passover. This view was first suggested to the modern world by French scholar Louis Duchesne in the early 20th century and fully developed by American Thomas Talley in more recent years. But they were certainly not the first to note a connection between the traditional date of Jesus’ death and his birth.
Around 200 C.E. Tertullian of Carthage reported the calculation that the 14th of Nisan (the day of the crucifixion according to the Gospel of John) in the year Jesus died was equivalent to March 25 in the Roman (solar) calendar. March 25 is, of course, nine months before December 25; it was later recognized as the Feast of the Annunciation—the commemoration of Jesus’ conception. Thus, Jesus was believed to have been conceived and crucified on the same day of the year. Exactly nine months later, Jesus was born, on December 25.