Complex sentence: In the summer of 2017 Phish preformed 13 consecutive concerts at the venue, which the Garden commemorated by adding a Phish themed banner to the rafters.
Split into simpler sentences: In the Summer of 2017 Phish preformed 13 consecutive concerts at the venue. At the conclusion of the thirteenth night The Garden commemorated this achievement by adding a Phish themed banner to the rafters.