Early spring bloom is best. That is simply how the schedule of the majority of flowers works. The priority of flowers is pollination. Pollination is necessary for the production of seed. The production of seed and any associated fruit takes time. Seed, whether contained within fruit or not, then disperses before winter. After soaking and chilling through winter, seed germinates for the next spring.
For a variety of reasons, some flowers prefer to bloom earlier, later, or randomly through the year. Some are from climates in which they want to avoid harsh weather of a particular season. Some rely on pollinators who are active for a limited time. Regardless of the reasons for their bloom schedule, early, late and randomly blooming flowers add color to the garden before and after spring.
Many flowers that bloom randomly through the year tend to bloom better and later with a bit of persuasion. Cutting roses regularly seems innocent enough, but actually deprives rose plants of their efforts to produce seed. So does deadheading to remove their developing fruit structures that contain seed. Plants respond by trying to bloom again or more prolifically than they would otherwise.
Lily of the Nile reliably provided much of the color through the middle of summer. Many gardens have some. Some gardens have many. Their color range is limited, but effective. Now that they are done, canna, dahlia and delphinium should continue to bloom until frost. Mexican blue sage that took a break after spring bloom should bloom even better as summer ends, and into early autumn.
The bloom schedule of many flowers of the Compositae (or Asteraceae) family also coincide with late summer. Some have been blooming since spring. Some just started recently. These include but are not limited to cone flower, blanket flower, zinnia, cosmos, coreopsis, sunflower and Japanese anemone. African daisy and euryops daisy often bloom well after the earliest rains of autumn.
Eucalypti that bloom colorfully, such as red flowering gum, Eucalyptus ficifolia, bloom after the warmth of summer, but before cooling autumn weather.