Anytime a freezing cold snap is predicted during the sprinkler season you need to protect the back flow device from freeze damage by either draining it or insulating it well.
Position all valves at 45 degree position! half open/half closed...

The beauty of the freeze tolerant ball valve is that if you forget to drain your valves, or an unexpected cold snap catches you off guard, NO BIG DEAL. It's a $5/5 min. fix!  You can get the replacement plug & tool online here ! Or call your sprinkler man for a cost much less than replacing a cracked valve.