Bread should be left open to dry out for a day.
Cut day old bread into 1/2" cubes.
If bread is still too soft, you can place on sheet pan and bake in 250 oven to further dry.
Place bread in a very large bowl and set aside.
Place butter, onion, celery, salt and pepper in medium skillet over medium heat.
Saute the onion and celery until tender (about 10 minutes).
Set aside to cool for about 10 minutes.
Preheat oven to 350.
Beat egg in a small bowl. Set aside.
Pour the vegetable mixture over the bread and mix well.
In a bowl or measuring cup, combine broth and poultry seasoning. Whisk to combine.
Pour broth mixture over bread mixture and mix well.
Pour egg over bread mixture and mix well.
Transfer stuffing to 9x13" baking dish.
Bake until golden brown (about 35-40 minutes).