Why does every year have a Friday the 13th?

A month has a Friday 13th if and only if it begins on a Sunday. read more

.The maximum number of days in a month is 31 .The number of days of a week is 7 .The minimum number of days in a year is 315 .31*7=217 .217 means there is all kind of combination of days and weeks .217<315 .Thus there is always Friday the 13th every year, not just Friday the 13th but all kind of dates as well. read more

It’s an artifact of the calendar. Proving it is an application of mod 7 arithmetic. A month has a Friday the 13th if its first day is Sunday. Let January begin on day k, 0≤k≤6 (with k=0 being Sunday) of a regular year. read more

i decided i need a property for Fri the 13th to make my proof easier. read more