It depends what the water temp is. If you live in a really cold climate and the water has gone below about 45 degrees F then the fish are hibernating and have no need to eat. You have to make sure they are really well fed during the warm months so they build up a good layer of fat to get them through the winter. 

BUT WHAT IF THEY DIE?WON'T THAT BE BAD?

I live in Minneapolis and have a plastic pond.  It's about 2 1/2 ft. underground.  I tried to take all the water out and found I still had a goldfish in it. What do I do with the fish during the winter? 

If the fish don't eat during cold months, don't feed them. Uneaten food will just foul the water. When the weather warms up they'll get more active and they'll get their appetite back.
If it gets really cold in your area (Snow etc), think about keeping them indoors over winter. A lot of people keep koi in their basements or garages over winter.

I live in BC Canada and we do get very cold winters, as a matter of fact each year the pond freezes solid.  As was said in another post feed them pleanty during the summer months, watch how much they eat by feeding smaller amounts first until all is eaten then add a little more until they stop eating this will tell you how much they will eat at each feeding.  I feed ours twice a day during the hotter period and as the weather cools you cut back the amoount making sure they eat it all. The goldfish will eventually eat very little to nothing as the cols sets in the pond will freeze and the are in hibernation until spring if you live in this type of winter.  In the spring the cycle starts all over.
PatC