Timed Selection Between AC Power Sources
K1 is a Double-Pole Double-Throw relay/contactor with a coil rated for your AC mains and contacts rated to supply whatever your freezer needs from the mains, and the timer - for example - is a 24 hour timer which can run off your mains and  be programmed to switch the mains to the relay coil in a never ending  cycle of 12 hours on and 12 hours off.