Fall is the best time to do most landscaping in the US, roughly September through early November, because the soil is still warm and plants put energy into roots instead of leaves. Early spring is the second-best planting window. Summer is the worst, since new plants need constant watering to survive the heat.
Hardscaping runs on a different calendar. Late fall and winter are when contractors have open schedules, and quotes often come in 10 to 15 percent lower than during the spring rush. Booking a spring patio build in January is normal in most markets.
Use the off-season to settle the design, since rushed plans are the expensive ones. You can work out the full layout from a photo of your yard over the winter with OutdoorBrite's design plans.