Melt butter in a large skillet over medium-high heat. Add celery and onions; cook 5 minutes or until tender. Remove from heat. Combine onion mixture, cornbread, and next 3 ingredients in a large bowl; mix well. Spread cornbread mixture into a lightly greased jellyroll pan.
Bake at 350° for 30 to 33 minutes or until set. Cool on a wire rack 30 minutes. Refrigerate 1 to 2 hours or until chilled.
Loosen edges of cornbread dressing with a sharp knife. Turn out onto a large cutting board. Cut into 3/4" cubes. Place in a single layer on greased large baking sheets. Bake at 450º for 35 to 40 minutes, turning occasionally, or until well browned and crisp.