Hi Rodd, And Welcome,
Since I didn't take time to look at your geographical location, I will give you my Best general advice! recommended time for digging and dividing older clumps is right after bloom. At this time new root growth is still active. However keeping transplants moist for the following 6-8 weeks is most important for successful establishment and hot dry weather may decrease survival. So this approach may work best in the cooler, wetter parts of the country. If watering is not always possible at this time, you may prefer early September transplanting when fall rains are somewhat more predictable and the sun's rays less intense.