The official purpose of Christmas is to celebrate the birth of Jesus Christ, whom the world’s 2.4 billion Christians worship as the Son of God. The unofficial purpose is to spend time and exchange gifts with loved ones, preferably near a warm fireplace. Based on the fact that Christmas (short for “Christ Mass”) takes place on December 25, one might reasonably deduce that this is the date of Jesus’ birth. The truth is a little more complicated.

First of all, no one can say with any real certainty precisely when Jesus was actually born. (Some even think it took place in the spring!) The timing is further complicated by the discrepancies between the Gregorian and Julian calendars, which is why Christmas isn’t universally celebrated on December 25; January 6 is the preferred date for Armenian Apostolics (fun fact: Armenia was the first country to adopt Christianity as its state religion), while most Oriental and Eastern Orthodox churches observe it on January 7.

One of the most widely accepted theories involves the Winter Solstice, which in Ancient Rome took place on December 25. According to the fourth-century theologian Augustine of Hippo, Jesus chose to be born the shortest day of the year: “Hence it is that He was born on the day which is the shortest in our earthly reckoning and from which subsequent days begin to increase in length. He, therefore, who bent low and lifted us up chose the shortest day, yet the one whence light begins to increase.” This interpretation was later supported by Isaac Newton.

Another theory centers around the Annunciation, or the day that the Archangel Gabriel told Mary she would give birth to the Son of God, which is observed on March 25 — or exactly nine months before December 25.

Beyond the date, the actual year Jesus was born isn’t agreed on either. There was no actual year 0, of course — remember that BC stands for “Before Christ” and AD means “anno Domini,” or “in the year of our Lord” — and most scholars place his birth between the years 6 BC and 1 BC.

Whatever the case, Christmas taking place on December 25 is hardly a new phenomenon: Constantine, the first Christian Roman Emperor, decreed that it be held on that date in 336.