The date of MARCH 21ST was selected by Down Syndrome International (DSI) to signify the uniqueness of Down syndrome because if you have Down syndrome, you have 3 number 21 chromosomes. The original idea was proposed by Stylianos E. Antonarakis, a medical geneticist of the University of Geneva Medical School. The first events were organized on 21 March 2006 in Geneva and the inaugural WDSD was launched on 21 March 2006 in Singapore, with events organized by the Down Syndrome Association.
And together, the world has celebrated "WORLD DOWN SYNDROME DAY on MARCH 21ST ever since. ;-)