The answer is…… it depends on where you live and whether you want to have flowers the first season after planting.
Generally, iris can be dug, divided and planted all year. Many people plant them in late summer and fall. At that time single rhizomes are the largest and the weather in most parts of the country is very suitable for the plants to settle in. Minimal intervention from gardeners is needed as fall brings plenty of rain and cooler temperatures.
In my garden, I divide and move them when I get around to it. The main disadvantage in planting too late in the spring is a delayed bloom time and first blooms may not look their best. Iris blooms not quite their best, however, still look awesome!