<P> The longest period that occurs without a Friday the 13th is 14 months, either from July to September the following year being a common year starting on Tuesday (F) (e.g. 2001 / 02, 2012 / 13 and 2018 / 19), or from August to October the following year being a leap year starting on Saturday (BA) (e.g. 1999 / 2000 and 2027 / 28). </P> <P> The shortest period that occurs with a Friday the 13th is just one month, from February to March in a common year starting on Thursday (D) (e.g. 2009, 2015 and 2026). </P> <P> Each Gregorian 400 - year cycle contains 146,097 days (with 97 leap days) or exactly 20,871 weeks . Therefore, each cycle contains the same pattern of days of the week and thus the same pattern of Fridays that are on the 13th . 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 days, whereas Thursday the 13th occurs only once every 213.59 days . </P> <Table> Distribution of the 13th day over the 4,800 months <Tr> <Th> Day of the week </Th> <Th> Monday </Th> <Th> Tuesday </Th> <Th> Wednesday </Th> <Th> Thursday </Th> <Th> Friday </Th> <Th> Saturday </Th> <Th> Sunday </Th> </Tr> <Tr> <Th> Occurrences </Th> <Td> 685 </Td> <Td> 685 </Td> <Td> 687 </Td> <Td> 684 </Td> <Td> 688 </Td> <Td> 684 </Td> <Td> 687 </Td> </Tr> </Table>

Where did the idea of friday the 13th come from