Heat the oil in a small skillet over medium heat. Add the onion, and celery, cover, and cook until softened, about 5 minutes.
Transfer the cooked vegetables to a 4- to 6-quart slow cooker. Add the sweet potatoes, squash, stock, thyme, and sage; season with salt and pepper, cover, and cook on Low for 6 hours.
Purée the soup in a blender or food processor, working in batches, or directly in the slow cooker using an immersion blender. Taste to adjust the seasonings, and serve hot.