I found many replies to my previous thread were little thinking
of it. Perhaps nowadays few people like the beauty of math.
Question: Every year has at least one Friday which falls on the
13th of a month, and at most three such days.
To prove: Any year could start with Sunday / Monday / ... Saturday,
and it could be a normal year, which has 365 days, or a leap year,
which has 366 days.
So we have 14 categories of years: normal year starting with Sun
till Sat; leap year starting with Sun till Sat. After we examine
all these 14 types we find that a year has at least one at most
three Fri. the 13th.
Please note that "every year" falls to one of the 14
categories.