Each Gregorian 400-year cycle contains 146,097 days (365 * 400 = 146,000 normal days plus 97 leap days), 146,097 / 7 = 20,871 weeks, and 400 * 12 = 4,800 months. Thus, each cycle contains the same pattern of days of the week (and thus the same pattern of Fridays the 13th), but no day of the month up to the 28th can occur the same number of times on each day of the week (because 4,800 is not divisible by 7). The 13th day of the month is slightly more likely to be a Friday than any other day of the week. On average, there is a Friday the 13th once every 212.35 (212 and 241/688) days.