Ah, the classic numerical systems joke that separates the nerds from the normies. For the uninitiated: Oct 31 is Halloween in our regular decimal system, but if you interpret "Oct" as octal notation (base 8) and "Dec" as decimal (base 10), then Oct 31 = 3×8¹ + 1×8⁰ = 25 in decimal.
It's the perfect joke to make your non-technical friends stare blankly while you snicker into your fifth cup of coffee. Just don't try explaining it at parties unless you enjoy watching people slowly back away.