I usually figure that if I can get my shovel into the ground it is a good time to transplant hostas and ferns.
Do it now and they will have a good start before the weather gets hot.

I am waiting another week or so to transplant mine. Sounds like we may get some frost, and I would rather not take any chances. I could be wrong, tho.
I have read that you are fairly safe after about May 10.

Linda

For ferns- I try to do it just before the fronds unfurl- or, later in the fall when broken fronds are not long for this world anyways...
I like to lift and seperate hosta as soon as I see their little eyes....
It is the best time to do the deed, in my opinion- no moisture loss to leaf surfaces- no broken, bent, or torn leaves all season- the soil temps are good for root healing and growth- no stress there either- I will plant them out almost any time- except very late in the fall. I have found there is a higher sucess rate if I leave all of those LATE season plantings to overwinter in their pots- and then plant out in the spring.
A very general rule of thumb for most perennials is- divide spring bloomers including iris after bloom time- divide summer and fall bloomers in spring.
I will move or divide almost all season- except when it is very hot out. The whole key to sucess, in my opinion, is to minimive the stress you put your plant through- clean cut tangeled root balls instead of squeezing and ripping apart- leave complete root ball intact whenever possible- repot or plant and water asap!
Best of luck to you!
Julie