Day of the Programmer, during a non-leap year. (Russia)

Programmer's Day, also known as the Day of the Programmer, is an international professional day that is celebrated on the 256th (hexadecimal 100th, or the 2⁸th) day of each year (September 13 during common years and on September 12 in leap years).

A leap year is a calendar year that contains an additional day compared to a common year. The 366th day is added to keep the calendar year synchronised with the astronomical year or seasonal year. Since astronomical events and seasons do not repeat in a whole number of days, calendars having a constant number of days each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("intercalating") an additional day—a leap day—or month—a leap month—into some years, the drift between a civilisation's dating system and the physical properties of the Solar System can be corrected.

Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the largest country in the world, and extends across eleven time zones, sharing land borders with fourteen countries. With over 140 million people, Russia is the most populous country in Europe and the ninth-most populous in the world. It is a highly urbanised country, with sixteen of its urban areas having more than 1 million inhabitants. Moscow, the most populous metropolitan area in Europe, is the capital and largest city of Russia, while Saint Petersburg is its second-largest city and cultural centre.