June 28th (hint: Pi Day is March 14th: 03 / 14 - double both numbers and you get 06 / 28)

Why is Pi Day celebrated on March 14th?

Pi Day celebrates the mathematical constant π (pi) - in the mm/dd date notation, 3/14, since 3, 1 and 4 are the first three digits of π. March 14, 1879 is also the birthday of Albert Einstein, and the two events are often celebrated together.