Once per year, the date and time become XXXX-03-14 13:59. Since 13:59 is commonly rendered as 1:59 pm, this becomes 3-14 1:59.
The first six digits of π are 3.14159, and through an English language pun, many take that moment, after lunch, to celebrate with "pie". It is a delicious math joke.
This cover, bearing stamps commemorating John Bardeen (Scott #4227) and Charles Steinmetz (Scott #2055) features the first several hundred digits of π, arranged and colored to depict a fruit pie with a lattice topping. "PI DAY" and two π symbols in the corners complete the cachet, and the cover was cancelled with a Pittsburgh, Pennsylvania "National Pi Day" pictorial postmark from March 14 of 2016.