Complex sentence: For many years Darlington was the site of two annual Sprint Cup Series races; one, the Rebel 400, was held in the spring and the other, the Southern 500, was always held on Labor Day weekend.
Split into simpler sentences: For many years, Darlington was the site of two annual Sprint Cup Series races. One, the Rebel 400, was held in the spring while the other, the Southern 500, was always held on Labor Day weekend.