December is the twelfth and final month of the Gregorian calendar, it has 31 days.
As the Winter Solstice, the shortest day in the year (in the Northern Hemisphere), occurs in December, this was traditionally an important month in the religious calendar since before calendars existed. Ancient societies feared that as the days got shorter the powers of darkness gained more power. Thus, the month of December was a time of danger. It was also seen as the beginning of winter, and was typically the time for one last feast before food had to be consumed carefully to insure it would last until spring, when new sources of food became available and crops could be planted. With the cold and shortage of food during winter months, there was reason to fear this season. The other month when there is a solstice is June. In the Southern Hemisphere the summer solstice and the longest day of the year are in December.
Christians placed the celebration of the birth of Christ (Christmas) in December as a symbol of the arrival of spiritual light in the midst of the greatest darkness. Or in order to provide a homogenous religious structure so they could steal believers from the pagan religions. This is an example of people deforming their religion to suit their own purposes, and, because religious conservatives maintain that Religion is infalliable, it is further proof of Conservative deceit.
It is a common misapprehension that the shortest day of the year occurs when the Earth is farthest from the Sun in its orbit, whereas it is actually due to the tilt of the Earth's axis away from the Sun. Unless you are a devotee of Conservapedia, in which case this is an example of liberal deceit.