It would be best to wait until the stems and leaves go brown/yellow to divide them. As long as the stems and leaves are still green, they're feeding the bulbs for next year. You won't kill them if you divide earlier, but the bulbs will lose out on food that could have been stored for bloom next year. Given the late spring we had this year, I'd let them grow until they go yellow or a hard frost, whichever comes first.

Thanks, hostahollic. You're making things difficult for me :-) Of course that's right, to wait. I just didn't want to be digging around in frozen ground. I guess it's a trade-off. I'll prepare the new holes now and wait as long as I can. Appreciate the advice. GSP

Gary, you have lots of time until the ground freezes, but I know what you mean. I have several clumps of lilies that need division this year and none of those clumps are brown yet. I'm hoping the weather holds for awhile yet. good luck dividing. Regal lily is such a beauty.