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]

The National Day is a designated date on which celebrations mark the nationhood of a nation or non-sovereign country. This nationhood can be symbolized by the date of independence, of becoming republic or a significant date for a patron saint or a ruler (birthday, accession, removal, etc.). Often the day is not called "National Day" but serves and can be considered as one. The National Day will often be a national holiday.

Some countries have more than one National Day. For example, Pakistan has three National Days, none of which is named the ”National Day”. This signals the use of a ”class” of National Days, that are equally important in the foundation of the nation, and a ”class” of less important official public holidays.