Frequently Anticipated Question:
Can you explain the Halloween/Christmas answer?
Sigh… Computer programmers work in a number of different number bases, such as binary (base 2), octal (base 8), decimal (base 10) and hexadecimal (base 16). To avoid confusion, they often indicate which base they are working in by prefixing their numbers with the first three letters of the base name (bin, oct, dec, hex). The decimal number 25 is 31 in octal, so Oct 31 = Dec 25.
It's actually a rather clever joke, but it's pretty much ruined if you have to explain it.
