Even though technically 2015 is the 11th time the event has happened it would still be the 10th annual.

quite the debate...
The way I see it is this. The very first time of the event is held is not counted in the annual calculation. So, if you started in 2005, you don't count that as an annual event This way your 10th year would be 2015. Otherwise if you count 2005, your tenth anniversary year would techincally be 2014. And to me 10 years of an event should be 2005-2015 (not 2005-2014).