Today, December 21, is the shortest day of the year which is known as the Winter Solstice. This date has had significance to human civilization for thousands of years. When humans first became civilized, their civilizations (as they still are today) were based on agriculture. The community would grow crops all spring and summer, and harvest and prepare for the winter all fall. The winter brought hardship, and the Winter Solstice was seen as a time of celebration. From this day on, every day is longer.

Ancient peoples based a number of religions around this and other astronomical dates on the calendar. It should not be surprising that the Romans also based their holidays on these celestial dates, including the Winter Solstice.
In 46 BC Julius Caesar introduced the Julian Calendar. This calendar was a refinement of the previous Roman Calendar. A year later it was officially adopted by the empire. The Julian Calendar remained in place for much of the western world until 1582, when it was replaced by the Gregorian Calendar. The Gregorian Calendar is what we use today.
Under the Julian Calendar, the Winter Solstice was on December 25th. This was a major holiday in the Roman Empire, just as it had been throughout much of the world spanning back perhaps three to four thousand years before Caesar. The Romans celebrated this holiday for centuries before the adoption of the Julian Calendar, and for centuries after.
Somewhere between 200 and 350ish AD (on our Gregorian Calendar) a newly invented religion known as Christianity also placed one of its major holidays on the Winter Solstice. This holiday, called Christmas, did what many other, previous religions had done and took traditions from previous religions. Many of the things we today associate with Christmas are from so-called Pagan festivals. Perhaps you have heard the saying “Yule-tide greetings!” Yule-tide was/is a Pagan holiday that also took place on the Winter Solstice. There are a number of other Winter Solstice artifacts from other religions and celebrations. The Christmas Tree, gift-giving, lights, and increased charity all derive from pre-Christian traditions of various cultures celebrating their Winter Solstice festivals.
When the Gregorian Calendar was adopted Christmas stayed tied to its December 25 time-slot, and the Winter Solstice became December 21 (and sometimes December 22). This effectively moved Christmas. You can move the calendar, but the Winter Solstice is based on the length of the day and so cannot be moved.
With this knowledge of the deep history and importance of the day, I wish everyone a happy Winter Solstice!
