Whoever first noticed this was pretty observant....
Q: Why do mathematicians often confuse Christmas and Halloween?
A: Because Oct 31 = Dec 25.
In case you don't get it, "Oct" refers to octal numbers and "Dec" refers to decimal numbers. Octal numbers are base 8, which means that as you count up, you carry to the next digit on 8, whereas decimal numbers are the ones that we're all used to where you carry to the next digit on 10. The first 10 positive octal numbers run: 1, 2, 3, 4, 5, 6, 7, 10, 11, 12. This is just a neat coincidence that 31 in octal works out to be 25 in decimal.