IMO spring is the best time to divide mums. Those divided in the fall may not survive the winter. I divide them soon after they have fully emerged.

I agree with MN winter sow Gal. I will divide most any thing any time- but the spring divisions seem to always suffer less set back with the mums. In fact- they usually put on a great show that fall! I will start dividing these as soon as I can tell what it is after it breaks dormancy. Just an inch or 2 tall.