Later this month (December 6), the 69th William Lowell Putnam Mathematical Competition will be held at universities throughout the United States and Canada. In commemoration, this month's problem is from a previous Putnam.

In the Gregorian calendar:

- A common year contains 365 days; a leap year contains 366.
- Years not divisible by 4 are common years.
- Years divisible by 100 but not by 400 are common years.
- Years divisible by 400 are leap years.

What is the probability that Christmas (December 25) falls on a Wednesday?