It's not what you think
When people are not talking about Christmas, they have been talking about
the Mayan apocalypse or 'end of days'. Apocalypse does not actually mean some large catastrophic doomsday scenario.
Apocalypsis is the last book in the Catholic canon, the Biblia Sacra Vulgata. It is a loan word from the Greek since the New Testament was originally written in Koine Greek, the language that was commonly spoken around the Mediterranean in the early centuries of the Common Era. It is the name of book containing the revelations attributed to Saint John the Apostle (if that is indeed his name), during his exile by Domitian on the island of Patmos. It is accurately translated in the King James version of the Bible as Revelations because that is what apocalypse means: revelations or an 'uncovering'.
Doomsday could be a suitable substitute as a term because it is derived from the Anglo-Saxon 'domes dæg' or 'day of judgement'. The Mayan end of days should really be called Judgement Day because that is the day in which all the guilty will be judged and time will end in the Bible. So......, the Terminator got it right and, this time, he won't be back.
Finally, the calendar most likely ends on December 21 because that is the shortest day of the year in the Northern Hemisphere and the Mayans were in Mexico. I wonder how short their days can possibly be or how cold but they are definitely in the north. Most pre-Christian cultures had some kind of winter solstice celebration to mourn the death of the sun and to hopefully bring it back. Therefore, the calendar ends on this day because it is 'year's end' for a culture that worshipped a solar deity. There is no mystery but some kind of revelation would be awesome.