The thirteenth of February fell on a Friday this year (1998). March thirteenth was a Friday also, as will be the thirteenth of November. So even though there are seven days of the week that the thirteenth could fall on, one fourth of them fall on Friday this year.

Does the thirteenth fall on Friday more often than other days? What day does it fall on most often? Least often? Or, given enough time, does the day of the week the thirteenth of a month falls on approach a uniform distribution?