What is the most and the least number of Friday the 13ths you can get in one year? In a 12 month period? Proven mathematically...
Now assume that the first friday in the year is on day
Now for count how many of is divisible by . The largest of these totals is the most Friday the 13th there can be in a standard year and the smallest is the fewest Friday the 13th there can be in a standard year.
Repeat for a leap year.