;
What is with these ancient machines?

Q: Why can't programmers tell the difference between Christmas and Halloween?

A: Because DEC25 = OCT31

Explanation:

DEC25 means Decimal 25 - so the number 25.
OCT31 means Octal 31 - which is 25 (for those who do not remember their math, octal is base 8).

Still confused? Well, lets count:

  DEC = OCT     DEC = OCT     DEC = OCT     DEC = OCT
   00    00      08    10      16    20      24    30 
   01    01      09    11      17    21      25    31
   02    02      10    12      18    22
   03    03      11    13      19    23
   04    04      12    14      20    24
   05    05      13    15      21    25
   06    06      14    16      22    26
   07    07      15    17      23    27