It's December 25th, the Birthday of Mithras, the hero god of bravery and honor and male fellowship worshipped mainly by soldiers of the Roman Empire during the centuries when Christianity was spreading throughout the Empire, centuries that saw periodic toleration and persecution of Christians by the Roman authorities before that religion was adopted by the Emperor Constantine and made the Imperial state religion.
It's important to this day mainly because the Birthday of Mithras definitely was celebrated by Mithraists on December 25th before the Roman Catholic church pegged that same day as the birthday of Jesus Christ, who, if you read the New Testament, was probably born in the spring under the astrological sign of Pisces, hence the symbol of the Fish used as code by early Christians. Interestingly enough, according to Mithraist traditions, Mithras' birth, maybe even virgin birth, was witnessed by shepherds.
The origins of Mithraism have been largely lost to history, but it definitely has elements of both Persian Zorastrianism and Indian Hinduism. Ironically, the best readable article I could find on the subject is written by the descendants of Mithraism's ecclesiastical conquerors in the Catholic Dictionary, which link is here:
Very, very basically, Mithras himself was a gift to earth and mankind from Ahura Mazda, the absolutely Good god of Zoroastrianism. Mithras conquered the Sun in battle and then made the Sun his loyal friend, and then slayed the Bull of Ahura Mazda(regretfully, on Ahura Mazda's instructions) from whose blood and flesh the world and people were created. It was an exclusively male religion, with secret mysteries and initiations from which women were excluded. By all accounts, few as they are, its members had a duty to be loyal to each other and to help one another, and there was certainly a hierarchy of some sort.
The Catholics' best guess is that Mithraism was introduced to the Roman Empire by legionaries who were stationed on the banks of the Euphrates in the First Century CE, and I find that to be quite plausible. For a couple of centuries, Mithraism was one of the primary rivals of early Christianity, which makes me think that the Catholic Church's use of Mithras' birthday as Jesus' was probably a shrewd political move to try to eclipse an important Mithraic festival day with one of its own, just as Christians incorporated many pagan holidays into their own saints' days at different times in different parts of the world.
The historical record is sketchy, but it is known that the Emperor Commodus, the psychopathic son of the great Emperor Marcus Aurelius, was publicly initiated into Mithraism to better glorify his own insane self(the portrayal of Commodus in Gladiator with Russell Crowe seems to be pretty spot on), and the Emperor Julian aka Julian the Apostate tried to elevate Mithraism to its former status when he renounced Christianity in the late 4th Century. Shortly after that, Mithraism, as well as so many other religions, fades from Western history with the triumph of Christianity.
It's probably not coincidental that the Birthday of Mithras coincided with other, older, celebrations which included much feasting and drinking and partying around the Winter Solstice; the Roman Feast of Saturnalia comes to mind and the old Druids probably had something going on about that time of year as well.
In that sense, the spirit of Mithras, with a lot of assistance from other traditions, has survivied the millenia. And Mithras doesn't seem like such a bad guy. So, next time you hoist a toast with that Christmas cheer and celebrate like merry gentlemen or gentlewomen, knock one back for old Mithras. It can't hurt, and it helps to hold on to traditions far older and more valuable than the current capitalization of this holiday season.