Spring and early fall are both good times to plant perennials. A useful rule of thumb: plant fall bloomers in spring, and spring bloomers in fall, so each plant has a full growing season to establish before it has to perform.
Fall planting is underrated. Cooler air, warm soil, and regular rain let roots establish with little stress, and those plants usually outgrow their spring-planted siblings the next year. The cutoff is about six weeks before your ground freezes; after that, wait for spring. In hot climates, fall is usually the better window overall since spring plantings run straight into summer heat.
Check your hardiness zone before buying, since "perennial" only means perennial where the plant survives your winter. To see how a new perennial bed would actually look against your house, OutdoorBrite generates planting concepts from a photo of your yard.