From astronomy to religion, the year’s shortest day brings enduring mysteries.
CHR Comment: National Geographic briefly describes two theories about the relationship between the winter solstice and the date of Christmas. A prevailing view is that Christians chose December 25 as the date to celebrate Jesus’ birth to offer a Christian alternative to celebrating the pagan Sol Invictus (Unconquered Sun) holiday that was timed with the winter solstice, the shortest day of the year. A more common Christian explanation is that the day was chosen since it is nine months after the day when the angel Gabriel announced to Mary that she would miraculously have a child who would be the Savior (Annunciation Day, March 25; Luke 1:26-37).
Source: Everything You Need to Know About the Winter Solstice