Mole Day is an unofficial holiday celebrated among chemists on October 23, between 6:02 AM and 6:02 PM,^[1][2][3] making the date 6:02 10/23 in the American style of writing dates. The time and date are derived from Avogadro's number, which is approximately 6.02×10²³, defining the number of particles (atoms or molecules) in one mole of substance, one of the seven base SI units.

Mole Day originated in an article in The Science Teacher in the early 1980s.^[4] Inspired by this article, Maurice Oehler, now a retired high school chemistry teacher from Prairie du Chien, Wisconsin, founded the National Mole Day Foundation (NMDF) on May 15, 1991.^[4]