It's
true - in the 400-year
cycle of the
Gregorian calendar, the most frequently-occuring date/day
combinations are
Friday the 13th, along with
Saturday the 14th,
Sunday the 15th, and all the
other days that would
necessarily have to be in the same month.
The opposite of Friday the 13th - the least occuring date/day combination - is Wednesday the 31st.