1. Prepare cornbread according to package directions for a double recipe. Let cool 30 minutes; crumble into a large bowl.
2. Melt 1/3 cup butter in a large skillet over medium heat; add chopped celery and onion, and saute 10 to 12 minutes or until tender. Stir in sage, pepper, and salt. Stir celery mixture and bread cubes into crumbled cornbread in bowl, stirring gently until blended. Add chicken broth and eggs, and gently stir until moistened; stir in oysters. Spoon mixture into a lightly greased 11- x 7-inch baking dish.
3. Bake at 350° for 50 to 55 minutes or until golden brown. Garnish, if desired.
Note: For testing purposes only, we used Martha White Cotton Country Cornbread Mix. To make ahead, prepare recipe as directed through Step 2. Cover with plastic wrap; cover with heavy-duty aluminum foil or container lid. Freeze unbaked dressing up to 3 months, if desired. Thaw in refrigerator 24 hours. Let stand at room temperature 30 minutes. Bake, uncovered, at 350° for 1 hour and 10 minutes to 1 hour and 15 minutes or until golden.