The Mayan calendar runs out on Dec 21st or 22nd.
That is to say, it restarts. However some people have interpreted it differently. It can either mean a new start, which means there must be an end first or a significant change. Even scientists acknowledge that the ending of the Mayan calendar is significant in its own way.
There was a theory about a planet hitting the earth. This was meant to be back in 2009 I think, but when it didn't happen they shifted it to coincide with the ending of the Mayan calendar. The planet that is supposed to hit us doesn't even exist. It was made up by one woman who started a website which then fuelled the interest of doomsday prophesisers.
Any planet or asteriod that was on a collison course with the earth as soon as Friday would have been spotted by now. In fact Nasa say that it would be visible with the naked eye. It isn't so we are not going to be hit by a planet or asteriod on Friday.
Which leaves some other happening that may or may not occur. The 21st is the winter solstice but that is as far as it goes astronomically-wise.
So the signs are that nothing exciting will happen.
There was a lot more panic about the millenium bug and that never happened either.
I get the panic, sort of, because it can feel scary when you think about the cosmos and about nature. These things are out of our control. Science can do a lot of things but it cannot prevent tsunamis or other natural disasters. It does not fully understand the universe and because we neither understand nor can control these aspects then we fear them.
The realisation that actually, we are pretty much meaningless in the grand scheme of things, that our lives, our memories and our entire beings can just be eliminated without a trace is scary. We all like to think that we have a purpose and that our lives have meaning and when you are reminded of the bigger picture (which is what this prophecy is doing) you can get frightened.
The Mayans are helping us to realise that our planet is fragile and that we are fragile. For all our intelligence, we are pretty ignorant and pretty vulnerable. We might think of ourselves as mini-gods but we're not. If the earth were to be destroyed tomorrow the universe would carry on as if nothing happened. We really aren't very important at all.
That is what is so scary.