Why December 25
Was Jesus really born on Dec. 25 in the year 1 A.D.? Probably not.
The year of Jesus' birth was officially set by the Catholic Church the year that was proclaimed to be 523 A.D. (Anno Domini, or "year of our Lord"). (Some non-Christians prefer the alternate designation "C.E.," for "Common Era".) Subsequent historians have placed the birth anywhere from a few years before this date to a few years after. Authorities also set the birth date several centuries after the fact.
While the Middle East doesn't see much of any snow, it does have a relatively cold and rainy winter season. Shepherds tending their flocks by night would undoubtedly have been doing so no later than autumn. According to Reggie Keith, the birth most likely took place in "the middle of September; around, and quite probably on, the Feast of Trumpets (modern day Rosh Hashana) when Jerusalem would have swelled from approximately 100,000 people to over 1 or 2 million people." (Hence, the little complication about no room at the inn.)
The earliest Christians appear not to have celebrated Jesus's birthday. But by the year 336 A.D., a Roman calendar mentioned Christ's birth being celebrated on Dec. 25. This happened to also be the time of year when non-Christians in Rome held feasts honoring Saturn, (their harvest god) and Mithras (their god of light). Some other parts of the Roman Empire at this time celebrated the birth on Jan. 6, the date now known as Epiphany.
Constantine, the Roman emperor who made Christianity Rome's official religion, set the official date for Christmas as Dec. 25 in or around the year 390. Some historians believe Constantine did this to combine Christmas with the Saturn and Mithras celebrations (and also with the cult of Sol Invictus, a form of sun worship that had come to Rome from Syria a century before). Those other Roman December holidays, and similar festivals elsewhere in Europe, all came on or just after the winter solstice, the shortest day of the year in the northern hemisphere. The basic message of all these solstice festivals is that winter is not forever; life continues; stay of good spirit.
Similarly, Christmas has come to express the message that hope and redemption are on the way; that in the darkest times of the soul, spiritual renewal draws near.