Blend brown sugar, honey, molasses, orange juice, and mustard together in a saucepan over medium-low heat until thickened, about 45 minutes. Remove from heat. Let glaze sit until flavors blend, 8 hours to overnight. Preheat oven to 225 degrees F (110 degrees C). Place ham in a roasting pan. Bake in the preheated oven, checking occasionally, about 10 hours. If ham begins to darken at that point, cover with aluminum foil. Continue baking, about 2 hours more. Increase oven temperature to 275 degrees F (135 degrees C). Bake, coating ham thoroughly with the glaze mixture every 30 minutes, about 2 hours more. Remove from the oven and let stand 15 to 20 minutes before slicing. Serve any remaining glaze with ham, if desired.