You can do it now if you want to - but you need to keep the sections well watered frequently afterwards up until late autumn. Otherwise, September/October, provided the ground isn't waterlogged or frozen.

If you want flowers next year, a minimum of 6 weeks before the expected first frost in your area. I start dividing mine when the first of the autumn rains come. They can be divided in spring, but they take a while to settle in and send up scapes. If you haven't done it before, YouTube has some good videos on how to do it.