For a standard year (repeat this for a leap year) list which day number corresponds to the 13th of each month these are .

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.

CB