Most wildflowers can be planted in fall, summer and spring, but it would also depend on where you live, what your climate is like and how you want to approach the meadow. Site preparation would be the same regardless of where you live and the previous post is great advice to follow. Season however is not the only consideration, but climate is a factor. Say for example you live in a warm climate such as South Texas, Florida or California, then you are free to plant basically anytime, except when the season is unusually hot.
Fall planting has a distinct advantage of getting early blooming and in this instance you could easily compare wildflower planting to grass seeds as you probably know that grass seeds do better in fall for early growth. One of the most successful flowers that grows exceptionally well during fall planting is the popular Black-eyed Susan’s.
Even though it is fine to plant in summer and you would much rather plant in summer than in winter to use an example, but please check your climate. If you live in extreme temperatures avoid summer planting as seeds simply might not germinate.
Most gardening happens in spring and that is probably why most wildflowers that are planted by individuals are planted. You timing here is important though as frost must be passed and another important factor is that you must plant immediately after you cleared you soil. If possible you clear the soil and plant in the same day. There is a reason for this and that is because the turned soil also have turned weed seeds present in the soil and it might have a head start on the flower seeds if you say for example wait a few days or a week. These weed seeds might dominate your wildflower seeds and you end up with wasted sowing.