when it is dormant, ie strictly and if the weather is seasonal, nov to end Jan Having said that, needs must and if you dig it up with plenty of soil and take great care of it after the move, then it is possible to do it anytime. If you do move one that is actively growing, a good prune will help it to survive the move as the roots will have less leaf to provide water and nutrient for and will be able to focus on their own regeneration
In addition to woofgang and Guinevere's good advice, be sure to water the rose profusely for a week or two after transplant. Having said that, don't water so much that water is standing at the base of the rose, but do water each day. Remember, all roses need a lot of sunshine to prosper...