At least once every 28 years. Possibly more often.
Why not once about every 7 years? There are only 7 days on which the 1st can fall each year. Except for adjustments for leap years, that day should be Friday once out of every 7 times, no?
In the long run, everyday of the year is a Friday one-seventh of the time (each day is equally likely to occur). In the short term this can be different, as leap years occasionally "skip" the Friday...
The odds of October 7 falling on a Friday are exactly 14.5%. In other words, it happens an average of 14.5 times per century. October 7 is slightly more likely to fall on a Friday than on a Monday,...
In any given span of 28 years, any given day falls on any day of the week four times. In this instance, taking a span from 1985 - 2012, October 11 fell on a Friday in the following years:...