Why do people think the world is ending on Saturday, May 21st?

since the beginning of time we have been predicting the end of the world its a primal fear remember y2k? now days its just away to make money if and when the world ends we wont know it and it will be unexpected. It is always possible that something will happen to end the world as we know it but the fact of the matter is no one really knows and it only gives us more reason to enjoys every moment we have because we are not promised tomorrow