Today I assume human had been celebrating april fools day 2009 times. As there is no real citation on when it start, why not i just assume that it started on the year after division(A.D) started. So human had been officially fooling other human being for 2009 times.
And again today, people around the world was playing with it. I not sure what most people will do but most likely i like to see it in youtube later on.
But for sure, just make today a normal day you jokes with your friend or family and please do not over joke as there is some consequences that might happen.