Only One

There were three in 2009. In 2010, there is only one Friday the 13th. Going back the last ten years, it turns out there is one Friday the 13th every year and three is the largest possible number of Friday the 13th in a year.

1 in 2010
3 in 2009
1 in 2008
2 in 2007
2 in 2006
1 in 2005
2 in 2004
1 in 2003
2 in 2002
2 in 2001

According to a table in Eric Weisstein's World of Astronomy, 13th is slightly more likely to fall on a Friday than any other day of the week. Unlucky or lucky? It is only Mathematics lol. :P