Complex sentence: On 13 December, One Direction performed on "The X Factor" final; their last televised performance as a group, before their hiatus, will be on "New Year's Rockin' Eve" on 31 December 2015.
Split into simpler sentences: On 13 December, One Direction performed on "The X Factor" final. Their last televised performance as a group, before their hiatus, will be on "New Year's Rockin' Eve" on 31 December 2015.