The Zombie Apocalypse is not real, just something the movies made up for effect. The Zombie Apocalypse generally happens in the future when the world is collapsing and people who die come back to life. For more on the Zombie Apocalypse please see: http://www.ask.com/wiki/Zombie_apocalypse.
There is no set date for the zombie apocalypse, but many believe it will happen in the near future. It is thought that zombies may be a contributing factor in the end of the world predictions for 2012.