A festival was first held in Spring of 2005. They did it again in spring of 2006. They thought perhaps it might work better in fall, so they did it again in Fall 2006, then fall 2007 and fall 2008.
Spring 05
Spring 06
Fall 06
Fall 07
Fall08
Will Fall 09 be the 6th annual festival or the 5th annual festival?
I’d say 5th annual, as it’s the fifth year.
For practical reasons, I’d call it the 6th annual festival. The number is intended to tell you how many times the festival has occurred. If it’s run by the Pedantic Society, it’s the “4th Annual Festival and 6th Overall Because We Had Semi-Annual Festivals in 2006”.
Why call it annual at all? Why not just say 6th festival?
Presumably the festival is conducted annually, except for that one year.
Myself, I’d call it the ‘6th Annual’ because it is the 6th festival, and is intended to be held annually. I figure the ‘6th’ refers to the past – it was held 5 times before – and the ‘annual’ refers to the future – everyone knows to expect it this time next year.
It’s an annual festival, and the sixth of its kind. So it’s the sixth annual festival.
Doesn’t work… Only one of the 2006 festivals was semi-annual. A semi-annual festival is either a festival which was half a year after the previous one, or half a year before the next one, but either way, it only applies to one of them. Otherwise, one could just as well say that both festivals were annual, since each one was a year away from some other one.
I suppose if you really wanted to have fun, you could call it the Fifth Annual Sixth Festival, or something.
Yeah, it’s actually a pereneal, or do they plant it from fresh seed each time? 
I’d say it’s not an annual festival, because they have had it twice in one year, so this is simply the sixth festival.
Well it would be conspicuous in its absence, I think? If you advertise that something is the 6th Festival and don’t say annual, the mind might go to thinking it’s been done 6 times intermittently over the last 40 years or something.
Then what about just calling it the “2009 Whatever Festival”? It hasn’t been annual regularly enough to deserve that term, and it hasn’t been around long enough for anyone to really care how many there’ve been.
Hey, if people aren’t going to check the dates on the medals won for Arm & Hammer Baking Soda or things like that (hint: 19th century), I don’t think that’s a big issue.