You should plant as soon as possible. I don't use mixes because some mixes have species I don't want, but there are some advantages to mixes for establishing a new meadow or prairie area. In nature, most native seeds fall on the ground in summer and fall. A few species hold onto the seed into the winter, but not that many. Some, like penstemons for example, need the cold stratification. Unfortunately, it's not easy to find specific info on germination of native seeds. In any case, with mixes, I think they're meant to be planted in the fall.

Here is a link that might be useful: Fall Planting

Thanks, Linda. I appreciate it.