This question already has an answer here:
Assuming Christmas day falls on the 25th of December every year, and using the Gregorian calendar, is it equally likely to fall on any day of the week? (With no prior knowledge about last year's Christmas day, or the year before, or the year before that...)
It would appear from a linked question, and a helpful answer that this is not true for 400 years (since it is not divisible by seven). But it is true for 2800 years (since it is divisible by seven). Is there a flaw in this thought process, or does this question inherently depend on the time period you consider?