If you plan to release before spring next year, than I would say you should do so right away while the weather is still warm. Make sure you release them in a location near where the mother was found, and/or where other garters of their species can be found currently That's very important for their chances of survival.

I'm with Steve on this one. If it's at all possible, they'd stand a better chance if you could take care of them until spring or early next summer, then release them. If you can't, don't delay. Find where other garters can be found right now, and release them there. choose the location carefully but don't wait until August is over!