I divide mine in fall. The "rule of thumb" is if it blooms before midsummer (end of June), then divide in fall. If it blooms after midsummer or into fall, you can divide in spring. This allows for sufficient re-establishment of the root system before the flowering cycle and minimizes disruption to the bloom cycle.

Both seasons will work, take a clump of soil with the plant don't take them back to bare-root and keep them well watered.