Complex sentence: As Harley's biggest UK and European tour since the 1970s, the tour was made up of over 50 dates between late September and early December, and was to be the main form of promotion for the album.
Split into simpler sentences: As Harley's biggest UK and European tour since the 1970s, the tour was made up of over 50 dates between late September and early December. It was to be the main form of promotion for the album.