It's been my experience that hostas are pretty forgiving. I think spring is the best time, but I've divided many a hosta during the summer and fall, and I have never lost one. They will look a little rough and lop-sided until next year if you divide them- tying all the leaves together before you start will help to prevent tearing them all up while digging and dividing roots. Best of luck!

I agree---hostas are very tough plants and can be divided and replanted just about any time the ground is not frozen. Mine have always looked a bit ratty for a few weeks, but seem to recover in a hurry. My problem is not so much when but how :) I think my husband had to use a hatchet the last time---they were totally clumped in one big root mass!