The breast of the turkey finishes cooking well before the dark meat is done.
To keep the white meat from becoming overcooked while the dark meat finishes cooking, you need to take a piece of foil and place it over the turkey breast.
This allows the juice from the dark meat to run down into the breast. keeping it moist.
Use freshly roasted dark meat for the big night and your guests will be clamoring for more more more!
One reason for this confusion is that a turkey contains a large amount of both light and dark meat.