Mole Day is an unofficial holiday celebrated among chemists on October 23, between 6:02 AM and 6:02 PM. 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 and the National Mole Day Foundation (NMDF) was founded on May 15, 1991. Many high schools around the United States, South Africa, Australia and in Canada celebrate Mole Day as a way to get their students interested in chemistry, with various activities often related to chemistry or moles.