October 23. A "holiday" made for avid chemistry fans, big and small due to the fact that Avagadros Number (6.02*10^23) is very important in that field. Mole day is celebrated on Oct 23 because of the 10^23.

As Pi day is celebrated by all the math geeks out there, Mole day is affectionatly welcomed by all of the science geeks, especially all the chemistry majors. It is celebrated October 23 and commemorates Avogadro's Number (6.02 x 10^23), which is a basic measuring unit in chemistry.