In my research I was unable to find any history on National Clams on the Half Shell Day. However I did find a few interesting facts:
Native Americans ate and strung clams like beads
Clams were used as currency
National Marine Fisheries pioneered clam farming in Milford, Connecticut in the 1930′s
Commercial hatcheries were growing by the 1960′s
Most clams today are farm raised
To celebrate this day visit a local restaurant that has a special on clams or prepare your own.