In a small bowl, sprinkle the gelatin over 1/2 cup of the apple juice; set aside. In a large saucepan, mix the cranberries, the remaining 1 1/2 cups apple juice and the sugar. Bring to a boil over medium-high heat, stirring often to dissolve the sugar. Cook until all the cranberries have popped, about 5 minutes. Reduce the heat to very low. Stir in the gelatin and continue stirring until the gelatin is completely dissolved, 1 to 2 minutes. Transfer the cranberry mixture to a large bowl. Refrigerate until cool and partially thickened (a spoon drawn through the mixture will leave a definite impression) about 2 hours. Or, place the bowl in a large bowl filled with ice water. Let stand, stirring often and adding more ice to the water as it melts, until the mixture is cool and partially thickened, about 45 minutes. Lightly oil a 5-cup mold. Stir the apple, celery and walnuts into the cranberry mixture. Pour into the prepared mold and cover with plastic wrap. Refrigerate until the mixture is completely set, at least 8 hours, or overnight. (The salad can be prepared up to 2 days ahead.) To unmold, fill a sink or a large bowl with hot tap water. Dip the mold, just to the top edge, into the water and hold for 5 seconds. Remove from the water and dry the outside of the mold. Remove the plastic wrap. Invert the mold onto a serving platter. Holding the mold and platter firmly together, shake them until the salad releases from the mold. Cut the salad into wedges and serve chilled. The recipe can be doubled to fit a 9-cup mold. Make ahead: The salad must be prepared at least eight hours ahead; it can be refrigerated up to two days.