https://www.polygon.com/2018/7/17/17583440/destiny-2-solstice-of-heroes-summer-holiday-400-power-triumph

Solstice of Heroes, which will start on July 31, is Destiny 2’s new summer holiday event. It’s also the only way to get 400-power armor before the Forsaken expansion releases on Sept. 4. The Solstice of Heroes represents a memorial and a celebration of the past year.

The event centers around the destruction of the previous tower and rebirth of the Golden Age. Essentially, it asks players to remember the events of Destiny 2’s base campaign. This is done in a few ways, namely the replaying of old story missions.