The number 256 (2^{8}) was chosen because it is the number of distinct values that can be represented with an eight-bit byte—a number that is typically very well known to programmers. Starting from zero, the 256th value represented by a sequential permutation of 8 bits is unsigned integer 255. 256 is the highest power of two that is less than 365, the number of days in a common year.

By the same logic, the color white was chosen because it represents a hex number with the largest value before overflow: 0xFFFFFF, so programmers worldwide wear white in celebration.

Further Informations: http://www.programmerday.info/