With plants and flowering, this has very little to do with calendar designations and much more with specific weather conditions. Very generally, an early summer bloom could mean any time from May into June; midsummer, late June into early August and late summer anywhere from the end of July to late September.

Plants don't know nuthin' about calendars and it is always a bit tricky guestimating when a specific plant is likely to bloom. An unusually warm spring could advance flowering on the early selections by several weeks, excessive summer heat (and drought) will shorten a bloom season and a cool summer can delay or extend it by several weeks. As long as you have a selection of plants that cover the various stages of the growing season, you should have a long progression of flower color.