Yesterday, according to the 2012 doomsdayers, was supposed to be the end of the world. Sorry, the world is still here, but don't worry, there will be a new proposed date soon in the queue to delight once again humanity. Interestingly, unless someone proposes an earlier one, Isaac Newton's prediction is the next one, with an estimated end of the world for 2060. I have the feeling that 2038, the year where all UNIX dates will flip back 1901, is another likely candidate. Remember Y2K ?
Considering the frequency of such announcements, we may be tempted to say that sooner or later someone is going to hit jackpot. James Randi, famous magician and debunker, uses the same approach: every morning, he puts a new business card in his pocket, with the words "I, James Randi, will die today" followed by the current date. Imagine the amazement of those who will find him dead. Reality, however, is different. In 5 billion years, our Sun will grow into a red giant and swallow our planet, an event that won't take place in one day. It will be a progressive change, so it will be hard to assign "the day when the Sun cooked us all".
If we limit ourselves to extinction (or really strong reduction) of the human species, this may indeed actually happen rather soon and quickly. Meteorites actually do hit the planet from time to time, and a large impact may trigger mass extinctions like it did in the past, but we don't have to wait from problems from above. We live in an overpopulated planet, in exponential population growth, and way beyond the carrying capacity in absence of fossil fuels. I think we should focus on the real problems of humanity, instead of Mayan calendars.