The punchline to one of many geek jokes. The joke goes, "Why do programmers confuse Halloween and Christmas?" The reply, "Dec 25 = Oct 31" ... Which, if you laugh, typically means you've been spending too much time converting Octal to Decimal. Way too much time. (Still funny. Tell it at a party, pick out the geeks.)

Ever had a computer programmer come trick-or-treating at Yuletide, or known one who decorates a tree in his living room while everyone else is cutting scary faces into pumpkins?

The reason is obvious:

OCT 31 = DEC 25

If you don't get it, you'll just have to read this and think harder.