^More the cycle comes to an end in the way any regular 12 month calender we produce does. A new year begins rather than time ending. It's the end of the 13th b'ak'tun of the Maya Long Count Calender, a b'ak'tun is made up of 20 katun cycles which is roughly 349 years. All that hapens in relation to the Maya is that we enter the 14th b'ak'tun.
Other theories such as Niburu/Plantet X colliding with earth on that date have been widely discredited, mainly by Nasa who say we'd have seen something by now.
There's the Pole shift, but no one really knows what effect will have, but I don't think we should worry about it.
There's the earth being in direct alignment with the sun and centre of the milky way galaxy, but due to how slow space is this has happened on the winter Solstice for a good few years now and nothing has happened.
There's the moving to a new astrological age, moving from the age of Pieces to the age of Aquarius. Though that's not really an end time scenario, though people do believe it could bring about a new level of conciousness. As this image shows
It's a new beginning rather than an end.
I think some believe we will somehow become more aware have a new understanding or even overstanding of our existence.
I'd like to believe in the last one, but I don't think I can bring myself to. Sadly I think nothing much will happen.