Complex sentence: But at the behest of a friend, legendary impresario Sid Bernstein, Sullivan booked the Beatles on February 9,1964, which was the most-watched program in TV history to that point and still one of the most-watched programs of all time.
Split into simpler sentences: But at the behest of a friend, legendary impresario Sid Bernstein, Sullivan signed the group. Their initial Sullivan Show appearance on February 9,1964, was the most-watched program in TV history to that point, and remains one of the most-watched programs of all time.