I am in zone 4/5 here in Utah and the best time to divide here is July and August. I have a bunch to thin and relocate and I plan on doing so at that time. Hope this helps. When all have stopped blooming then you can start to divide them.
I have some that are still at it. July I begin.

since they have all finished blooming here near ann arbor ... now is the time ...

also.. i just bought some new ones from the iris lady down the road.. and planted them... ergo.. she thinks its a good time to plant ...

so the bottom line... whenever you have time between now and when you get it done.. but they need time to grow the roots in fall.. so dont wait too long ...

ken

thanks everyone, guess we start this weekend

I never start until July. Mostly because I was once told that they need time after blooming to just grow, but I know you can dig them even if they're in the middle of blooming and they do just fine. I try to have it all done by August, but even the start of September gives them enough time to establish before it gets cold here. Or at least cold enough to be a problem, considering that we can get snow in September here, but that's usually a warmer snow and doesn't bother the plants at all.