A false rumor is floating around the internet that this happens only once every 823 years.
The reality is that it happens whenever the first day of October is on Friday, which happens on average once every seven years.
The last time for this "rare" event was 6 years ago, back in 2004. And the time before that was 5 years previously, in 1999.
About twice each year
4 years
The month of July has 5 Saturdays and 5 Sundays zerotimes in 2012.
Over the very long average, once for every seven years. And it's "often".
This happens on average of once every 7 years- 4 times in every 28-year cycle of the calendar. It happens whenever March first is a Friday..
Exactly 14% of all Decembers have five of each of Fridays, Saturdays and Sundays.
Every year has more than 5 Fridays, 5 Saturdays and 5 Sundays.
March
ummm...never.
About twice each year
every 823 years.
There are 14 calendar variations (7 days of the week plus leap years). Some years October will have 5 Fridays, 5 Saturdays, and 5 Sundays. October 1999, 2010, 2021, and 2032 all have 5 Fridays, 5 Saturdays, and 5 Sundays. This looks like an 11 year gap, but October 1988 and 2043 do not have 5 weekends.
Never.
once every 365 years!
Every time February 1st falls on a Monday !
4 years
This happens in any 31-day month starting on Friday (such as July 2005).