Best is after they go dormant. Just mark the spot where it is and dig it up after the foliage has faded away.
If for some reason you can't wait that long, at least wait until it is finished blooming and preferably a little while after that--give the foliage time to feed the root, to replenish from the effort of blooming.
I would take Rainbows advice on waiting until they are dormant as they don't like to be moved to begin with and have very deep, fleshy, and easily broken roots.