https://www.gamespot.com/articles/destiny-2-servers-down-heres-when-forsaken-release/1100-6461388/

Update: Destiny 2 servers are now offline for pre-Forsaken maintenance. Additionally, update 2.0.0.1 is now available for download ahead of the game coming back online. Bungie says this should happen at around 10 AM PT / 1 PM ET / 6 PM BST; whenever it does, you'll be able to log in and begin diving into what Forsaken has to offer, provided you've purchased the new DLC. One aspect of Forsaken you won't be able to jump into immediately is the Raid, which is scheduled to debut on September 14. The original story follows.

Bungie has released detailed information about the launch for Destiny 2: Forsaken. The expansion comes to all platforms on September 4, but you'll need to download a Day 1 patch before you can play.