From The Old Farmer’s Almanac:
December 25 — Christmas Day. The ancient Celts divided the year into four sections marked by Quarter Days — the days of the two solstices and two equinoxes, on which the seasons begin. Gradually, to conform more closely to the liturgical year of the Christian church, the Quarter Days became identified with the church’s high seasonal festivals, which occurred close to the astronomical dates. Christmas, the fourth Quarter Day, was both the culmination of the old year and the first festival of the new year. The day originated as a solstice festival and signaled a time of resting and gathering fertility for a new round of sowing and reaping. This festival merged easily with the Christian celebration of the birth of Jesus Christ, which occurred at this time of year.
May Your Days be Merry and Bright.