In punch bowl, combine white grape juice cocktail, concentrated cranberry juice cocktail and water. Gently stir in ginger ale. To prepare ice ring, combine frozen grape juice with water and mix well. Place lemon slices in bottom of 8-inch ring mold. Pour small amount of grape juice over slices and allow to freeze. Add remaining grape juice and freeze solid. When ready to serve, unmold ice ring and place in punch, lemon side up. Serve immediately. Makes about 30 half-cup servings.