DNA Day is celebrated on this day, 25 April, and it is to celebrate the discovery of the double helix by scientists including James Watson and Francis Crick in 1953 (so 60 years ago today). On this day they published a paper in the Nature science journal describing the discovery.
Also, it was on 25 April that it was announced that the Human Genome Project for DNA sequencing, based on the discoveries of Watson, Crick and others, was to all practical purposes complete. That was 10 years ago in 2003. (It may have been carefully timed to coincide with the anniversary of the original discovery, of course).
Therefore, in the US there was a celebration on 25 April 2003, organised by the House of Representatives and the Senate. Ever since then, the Human Genome Research Institute has held its own celebrations every year.
You can find out more about DNA Day here.