Pi Day T-shirts are designed to celebrate Pi Day, which occurs on March 14 each year. Pi Day is a celebration of the mathematical concept of pi.
Pi Day is celebrated on March 14 due to the date's similarity to the mathematical number of pi. The numeric date, 3/14, sounds similar to pi's decimal expression of 3.14159, according to PiDay.org.
Pi is symbolized with a Greek letter and is used in mathematics to represent the irrational number that is the constant ratio between a circle's circumference and its diameter. The actual number has been calculated to over 1 trillion digits beyond its decimal point, according to PiDay.org.