It depends on the type of flower. If your flower seeds come in a packet, the packet will show a map you can find your general location on. Take that number and look it up on the table they provide and the table will tell you the best time to plant those particular seeds.
If you have wildflowers native to your area, the best time depends on when those wildflowers bloom. Spring and summer bloomers can be planted in late summer or fall. Fall blooming flowers should be planted in the spring.
It depends with the type of flowers seed you want to plant and the temperatures experienced in your area. Most of the flower seeds are planted during late winter and spring seasons. It is best you research more on the flower seeds catalogs.