Flower season allows you to understand where in the season a given cultivar's flower sequence falls. For those with very late spring freezes, early-early cultivars may be problematic, unless they have been selected for resistance to late freezes. In hot, southern gardens, late season flowers may be problematic.
I use the following seasons in my own program - early-early, early, early-mid, mid, mid-late, late, very late.
In my garden that sequence is typically as follows:
early-early - Late May through the first two weeks of June
early - First two weeks of June and on into third to fourth week of June
early-mid - Third to fourth weeks of June, perhaps extending into the second week of July
Mid - First two weeks of June extending to the end of June
Mid-late - Fourth week of June, extending into third week of July
Late - Third to fourth week of July into mid-August
Very late - August or later
They may bloom in different weeks in locations further north or south than my zone 6 garden.