Knock Out roses are hardy down to USDA Zone 5. They can be planted outdoors in USDA Zone 7 (average annual minimum temperature range of 0 degrees F to 10 degrees F) in the spring after the last hard frost and in the fall, up to six weeks before the first hard frost. There are advantages and disadvantages to be weighed for the timing of planting that largely depend on your climate. In any case, it's best to avoid the heat stress of the peak of summer or the cold temperatures and hard-to-work soil of winter in most climates.
Plant Knock Out roses in Zone 7 in early spring, after the last hard frost has passed. Spring soil is often wet and muddy, and rainy conditions weigh against spring planting. In climates where there are minimal spring rains, this is of little concern; in climates where spring rains are heavy, planting can be done on a dry day. Spring planting as early as possible (without frost) makes for a long growing and blooming season for Knock Outs, which will continue to bloom in cycles until the first hard frost of winter. Spring-planted Knock Outs will begin to grow again in two to three weeks after planting and come into bloom in six to eight weeks after that. Because spring-planted Knock Outs have the entire summer and fall to establish their root systems, they're well poised to survive winter undamaged.
Plant Knock Out roses in the early fall in Zone 7. Planting Knock Outs in September, October and even the first week of November can be done safely. This allows the roots to become settled before the cold of winter. Fall planting also holds the advantage of a drier climate and better soil condition, which can reduce mildew and rust on the rose--though Knock Outs are known to be fairly disease resistant anyway. Roses planted in the fall are also more likely to bloom earlier in the spring because they're already well established. The downside of fall planting can be that the selection of roses available in your garden center and direct from growers is much smaller, unless you reserved or pre-ordered them.