Wildflower Mixes composed primarily of perennials need time to establish before the onset of winter so plant in the early spring while weather conditions are still wet or plant in the early autumn when the rains come again. The summer is a good time to sow wildflower seeds also but irrigation will be necessary for germination and seed success. It's better to let nature do it's work and sow during rainy periods. Sometimes that is the case during the summer!
Annual Wildflower Mixes should be planted in the spring so they have time to flower and set seed by autumn.
Decisions of when to plant and where and of how intensively to manage the site until it is established are contingent upon too many factors to fully describe in this space. Use your instincts. Observe the site as it progresses. Does it need watering? Should you hand weed? Do you need to reseed?