Can you figure out why Christmas does not fall on a Friday exactly 1/7 of the time?

Granted it is awfully close, but not exact. There is nothing calendrically unusual about Christmas. I chose it because Friday on Christmas means a 3 day weekend.

If there were no leap years, Christmas would occur on Friday exactly 1/7 of the time, and even if leap years always occurred every four years. But the rule for leap years is slightly more complicated. Every 100 years is not a leap year, except that every 400 years is a leap year. I imagine people would be more aware of this if the new millennium, 2000 had not been divisible by 400.

A simple arithmetic calculation shows why 1/7 can’t possibly be right.