Ducky Day was an annual fun day for Cray employees. During Ducky Day there were organized sports, social events, stuff like that. There was also a competition to create the T-Shirt for that year's Ducky Day.
The origins of the name "Ducky Day" came from a practical joke. Outside Cray's 1440 building there was a fanciful pond with a sculpture in it (I believe this is "Cray Lake"). One employee had placed a plastic duck in the pond which result in a stern warning from the management about it. In retaliation, the entire pond was filled to the brim with ducks the next day.