Late spring and summer are the best time to add fish to a pond. Adding fish to your pond in the midst of winter can be too stressful on their bodies, as they wont have the fat stores necessary to survive the period without food and will likely not be in a dormant state.
If you already have your fish, but the water is too cold for them to be added to your pond, its reccomended to keep them indoors until spring arrives. This protects vulnerable fish from extreme cold and lack of resources that comes with wintertime.
