Preheat the oven to 375 degrees F.
Mix together the carrots, celery, zucchini, onions and squash in a large bowl. Coat a large roasting pan with the olive oil. Spread out half of the vegetables in the pan. Scatter with half of the garlic and season with 1/2 teaspoon each salt and pepper. Add the other half of the vegetables in a second layer, scatter with the remaining garlic, and season again with 1/2 teaspoon each salt and pepper. Cover the pan with foil and roast until the vegetables are tender, 25 minutes.