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.
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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..