Today is Annunciation Day. You can read the account of Gabriel’s announcement to Mary in Luke 1:26 and following.

Additionally, Biblical Archeology Review posted a helpful explanation as to why the birth of Jesus Christ is celebrated on December 25. Many have, sadly, bought into the quasi DaVinci Codesque conspiracy theories that Christians were trying to co-opt, ruin, or take over Saturnalia or Sol Invictus. Neither is true. Rather, the birth of Christ is remembered and/or calculated because of Annunciation Day, March 25. For those who ask why Annunciation Day? Their question is answered when they count nine months from March 25th.

Here’s a portion of the article on why December 25.

The December 25 feast seems to have existed before 312—before Constantine and his conversion, at least. As we have seen, the Donatist Christians in North Africa seem to have know it from before that time. Furthermore, in the mid- to late fourth century, church leaders in the eastern Empire concerned themselves not with introducing a celebration of Jesus’ birthday, but with the addition of the December date to their traditional celebration on January 6. 

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. 

This idea appears in an anonymous Christian treatise titled On Solstices and Equinoxes, which appears to come from fourth-century North Africa. The treatise states: “Therefore our Lord was conceived on the eighth of the kalends of April in the month of March [March 25], which is the day of the passion of the Lord and of his conception. For on that day he was conceived on the same he suffered.” Based on this, the treatise dates Jesus’ birth to the winter solstice.