It's best to divide them in fall, but it can be done in early spring too. I don't know what zone you are in, but I'd need a jackhammer to get through the frozen ground here.

Yes, as hostaholic said, they are best divided in fall, but you can actually divide them anytime as long as you don't break the new shoots.
People have posted that their broken new shoots would grow back, but mine never did. Any new shoots I broke off in spring, were all goners.