This is a question we get asked ever season. the answer is this:

As long at the pool filter is running below 38 degrees, the pool can be opened! Running water does not freeze nearly as easily as still water, plus the heat from the running pool motor will keep the water warm as it passes through. So, get em open early and beat the mad rush!! Keep this in mind, The "freeze protection" setting from the factory on automated filter control systems is set at 38 degrees. if the system is off and the temp dips down below 38 degreess, the system "freeze protection' program by passes the timer and turns on the system to protect it from freezing!