It would be best to divide in early fall due to less intense heat and usually more rain which will help the divisions get established better.
I just divided some black-eyed susans and day lillies a couple of weeks ago. They looked sad at first but are doing great right now- of course we've had lots of rain :( so I suggest If you do it now, they need to be watered regularly to recuperate. Good luck!
If there's an option, I would would do as Tammy recommends and wait until early fall.