Camping is basing his prediction on a mathematical system used to interpret biblical prophecy. He has suggested the world will end on 21 May, because that will be 722,500 days from 1 April since AD33, which he believes was the day Jesus Christ was crucified. The figure of 722,500 is important because you get it by multiplying three holy numbers (five, ten and seventeen) together twice.
If I'm being honest, I shan't lose any sleep tonight. I fully endorse the view of Calvin L. Smith who writes:
[T]he fact Harold Camping doesn't have the best of track records on this issue, together with Jesus' words in Matthew 24:36, would rather suggest we'll still be here tomorrow evening.Equally worth noting is Camping's reliance on a system he developed to interpret hidden prophecies in the bible. Surely, belief in a personal God who has anything to say to people also necessitates a belief that He has made Himself clear and understandable. Indeed, what purpose would there be in a revelation that could not be understood? Reliance on secret systems and hidden prophecies suggests God has not made his Word plain and that He seeks to speak in coded language intended only to confuse. Camping's reliance on such hidden messages belie the very concept of a personal God; a concept necessary for his prediction to be of any value in the first place.
So, I shall sleep soundly tonight and fully expect to wake up as usual the following morning. Of course, that may just mean I missed the rapture...