From Destinypedia, the Destiny wiki
Solstice (previously Solstice of Heroes) is a limited-time Destiny 2 event. It is typically the final event of the current expansion year, intended to celebrate all our accomplishments during that year.
Solstice is held in the Last City. It memorializes the Tower and celebrates the return of the Traveler after the events of the Red War. During this event, Guardians go to the European Aerial Zone for combat and meditation, fighting with various Cabal, Fallen and Hive bosses who seek to claim it.