The Myan's calander stopped because the Spanish wiped them out......
When the End of Days comes, you'll know it. But I'm fairly confident it won't be in 2012.
The closet thing I can predict to a "apoccalypse" is a nuclear holocaust, or when our government falls out, weather it be due to revolt, water shortage, fuel shortage, economic turmoil, or robotic domination.
Though we will likely end up the same way as after Rome fell (the Dark Ages), I suppose its for the better though, the worlds getting pretty pathetic.
Also about aliens.....
E.T.= God's side projects