Contrary to what you've been hearing on the internet this is not a rare occurrence, and it certainly happens more often than once every 823 years.
This is part of a cycle: In August 2009, there were five Saturdays, five Sundays and five Mondays. August 2011 will have five Mondays, five Tuesdays and five Wednesdays.
The next time we get five Sundays, Mondays and Tuesdays in August will be August 2021. These are things you can look up for yourselves, so there's no need to believe everything you get in your email inbox.
(Sources: 1, 2)