Programmer's Day celebrates the contributions of programmers to humanity. The day is unofficially celebrated on the 256th day of the Gregorian calendar year. Therefore, it usually falls on Sept. 13, although on a leap year it is on September 12.
The 256th day was chosen because 256 is 2 to the 8th power, which is the number of values in a single byte of data. In addition, 256 represents the point at which an 8-bit byte rolls over. As a result, it is reminiscent of the days of 8-bit computing.
Typical celebrations of Programmer's Day include coding silly programs, playing around with old computers and behaving in a generally inane manner -- provided, of course, that for the rest of the year the programmer has been industrious and produced well-written, bug-free code.