I am in your zone and region. I have settled into a routine of waiting until after the daylilies bloom, late August or early September, to assess which clumps would benefit (or not suffer) from dividing. Let the flowers guide you. And also your need for new divisions. Do you want to speed it up? Increase a particular variety? End of summer is also a slower time in the garden. I find early spring so frantic as it is.

Actually I'd like a few of them to speed up, any divisions I give a way. The end of the summer sounds like a much better time to divide any because I'm also so busy in the spring. What's your guide that would indicate they're truly ready to be divided? Thanks!

They just don't flower as profusely, especially in the center of the clump. And the rhizomes are an impenetrable mat.