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 1½ 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 apple juice with 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 dragged through the mixture will leave a definite impression), about 2 hours.

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.

To unmold, fill a sink or large bowl with hot tap water. Dip the mold, just to top edge, into the water and hold for 5 or 10 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 the platter firmly together, shake them until the salad releases from the mold. Cut the salad into wedges and serve chilled.

Note: The salad must be prepared at least 8 hours ahead; it can be refrigerated for up to 2 days before serving.