Instructions
Set your GameCube's internal clock to the month of December. To do this, turn on your Nintendo GameCube without any game in it. Click on the "Calendar Screen" menu option. Adjust the "Month" option to December and save your settings.
Load the Animal Crossing game and go into your town. Your town should be in the middle of winter if you did the previous step correctly.
Walk around the town until you spot a snow pellet. Walk up to the snow pellet and push it around for about 15 seconds. It will grow in size and become a snowball!