A complete cycle consists of the numbers 0 - 6, where each number is representing a specific day of the week. As we can see by the calendar, each day of the week falls on the thirteenth at least once during the calendar year. Some weekdays only occur once; when 13 is congruent to 0, 1, or 3. Others occur twice; when 13 is congruent to 4, 5, or 6. One day of the week occurs three times throughout the year; when 13 is congruent to 4. Since each weekday in the cycle is represented, we know that Friday the 13th must occur at least one time and at most three times throughout the year.