When a long spell of cold weather or freezing temperatures are expected it's a good idea to protect a water butt by draining it. When temperatures drop, the water inside the water butt freezes and as it does so it expands. The result is shown in the image left. The frozen water expands to the point it splits the plastic and the entire water-butt breaks.
I was caught out by the sudden, very cold weather and the water butt fractured. An expensive oversight. To avoid this problem, before the bad weather strikes disconnect and drain the water butt, and leave it disconnected until the worst weather has passed. To freeze a large solid a mass of water requires a spell of sub-zero temperatures, which can be experienced in December & January with temperatures remaining below freezing for many days. Simply re connect the water butt in the early spring.