Sep 13
Programmers’ Day is an unofficial international holiday celebrated on the 256th (0x100th) day of each year, which is September 13 (or September 12 in leap years).
On September 11, 2009, the President of Russia Dmitry Medvedev recognized Programmers’ Day as an official Russian holiday.
The number 256 was chosen because it is the highest power of two that is less than 365, the number of days in a common year. 256 is also the number of distinct values that can be represented with an eight-bit byte, which is a common unit of measure for computers and software.
Source: Wikipedia