Why not celebrate Christmas in September or October, the months in which Christ was most likely born? Because at the time that the official date of Christmas was being decided by the Church, the Romans were concluding Saturnalia on/around December 25, at which time each community killed an innocent person that they had selected to indulge in food and physical pleasures. The thought was that they were getting rid of dark forces.
At first, the Church tried to create a companion festival to Saturnalia in hopes of converting people during the festival.
Because of the pagan origins of the Christmas celebrations, it was illegal to celebrate Christmas when Puritans controlled the Massachusetts colony in the late 1600's.
By the way, lunar cycles and the Spring Equinox are how the date of Easter is traditionally determined - another example of setting holidays solely to coincide with traditional pagan holidays rather than around the days that the events celebrated most likely occurred.
Maybe, we should consider common Christmas decorations.
The Christmas tree comes from a ritual performed by a pagan cult at the time of the establishment of the Church's Christmas holiday. This cult specifically called these trees Asherah trees, which are also specifically prohibited in Old Testament law.
Christmas gifts trace their origin to Saturnalia, where an individual received gifts as described above.
Wreaths also come from Saturnalia and were decorated with images of Saturn for worship.
The Advent tradition of lighting candles on the wreath to symbolize the First, Second, etc Sunday of Advent traces its origins to wreaths being used by European tribes to celebrate the never ending cycle of seasons on Earth. The four candles on the wreath were lit to symbolize each of the four seasons, with a fifth in the center being lit to celebrate the persistence of life through Winter.
