Cut acorn squash in half lengthwise; remove and discard seeds and membranes. Place squash cut side down in a greased 11x7-in. microwave-safe dish. Microwave, uncovered, on high for 4-6 minutes or until almost tender.
Meanwhile, in a small bowl, combine the apple, celery, raisins, brown sugar and pumpkin pie spice. Turn squash over; fill centers with apple mixture. Cover and microwave 3-4 minutes longer or until squash and apple filling are tender.