Interesting. How is that possible? A 31-day month is 4 weeks + 3 days long, which means those last three days are repeats of the first three days of the month. So if the month starts on a Saturday, there will be a fifth Sat/Sun/Mon at the end.
You would think that a 31-day month starting on a Saturday would occur more often than once every 823 years... especially considering that there are seven 31-day months in each calendar year.