Complex sentence: Their first hit record was "That'll Be the Day", released in 1957, which became the number-one hit single on the "Billboard" Hot 100 chart in "Billboard" magazine on September 23.
Split into simpler sentences: Their first hit record was "That'll Be the Day", released in 1957. The single became the number-one hit on the "Billboard" Hot 100 chart in "Billboard" magazine on September 23.