When to plant shrubs depends largely if you are buying bare-root shrubs or pot-grown ones.
Cheap bare-root shrubs are available in fall (autumn) and spring. Many gardening books say that you can plant bare-root shrubs in fall, but even in my mild climate on the south coast of Ireland I have lost half of my newly-planted shrubs to frost over their first winter.
Bare-root shrubs are best planted in spring. Surface frost should not damage them, as long as the ground is easy to dig and not water-logged.
You can plant pot-grown shrubs at any time of year, but I do all my planting in spring. Some plants are sold as pot-grown but when you tap them out of the pot you find a puny bare-root shrub with no established root system that has been stuffed into a pot with a bit of loose compost.
Check underneath the pot for roots growing out. This is a sign of a real pot grown plant.
If you are buying large, root-balled shrubs these are too expensive to risk planting except in spring, even though they should grow successfully if planted at any time.