Christmas Day and the Winter Solstice Christmas Day is a Christian holiday commemorating the birth of Jesus Christ. Although the actual date of Christ's birth is unknown, it has been celebrated on December 25 since the 4th century. In ancient times, Celts divided the year into four sections marked by "quarter days"—the days of the two solstices and two equinoxes. The winter solstice, the shortest and darkest day of the year, was the fourth quarter day. In 2012, the winter solstice arrives at 6:12 A.M. On December 21 (EST), the earliest winter since 1896. The winter solstice signaled a celebratory time, as the Sun began to reemerge and the land experienced a rebirth. Gradually, the fourth quarter day merged easily with the Christian celebration of the birth of Christ. As Christianity began to spread in the 4th century, the Christmas feast day was set on December 25 by Pope Julius I to align with the Roman pagan holiday Dies natalis solis invicti, "the birth of the invincible Sun." Today's rich mosaic of Christmas customs dates back through the ages. Evergreen branches were used to symbolize life in ancient solstice festivals, as they stayed green in winter. Christmas lights may have evolved from the Yule log, which was lit to entice the Sun to return as part of the jól (Yule) festival in ancient times. Read more about Christmas facts and trivia—as we answer Christmas questions from our readers.