Programmers’ Day – Wikipedia.

Programmers’ Day is an international professional holiday, recognized in many technology companies and programming firms, 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). It is also officially recognized in Russia and observed in several other countries, including Austria, Germany, Canada, China, Croatia, France, Guatemala, India, Belgium, Australia, New Zealand, Poland, Slovenia, the United Kingdom, and the United States.

The number 256 (28) was chosen because it is the number of distinct values that can be represented with an eight-bit byte, a value well-known to programmers. 256 is the highest power of two that is less than 365, the number of days in a common year.