Tomado de WikiPedia:
"Programmer's day is a "holiday" on the 256th day of the year celebrated mostly by computer programmers (reason: 256 = 2 to the power of 8 = the number of values representable in a byte of data).
Traditions include drinking, behaving silly, coding silly programs, mini computer games, playing with old computers, etc. That, of course, includes the condition that the developer who chooses to celebrate, needs to have done decent coding on all the other days and had tried the best not to behave silly on the other days. For those programmers who are used to the 'celebrations' in their every day lives, this 'holiday' becomes unjustified!
Programmer's day usually falls on September 13th; on leap years, it is September 12th."