Does a Friday the 13th occur every year? How can you be sure?
Yes, Friday the 13th occurs every year. In fact, you can have a year with two Friday the 13th's. This can easily be seen by listing out each date of a given year. You'll notice that dates 1-7 will occur at least once during the months for the first friday. Thus, there must be at least one friday which is on the 6th. Using this same logic, there must be at least one friday which falls on the 13th. You can show this for both leap and non-leap years.