https://www.gamespot.com/articles/heres-when-destiny-2-forsaken-and-its-day-one-upda/1100-6461388/

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.

On September 4 at 7:00 AM PT / 10:00 AM ET / 3:00 PM BST, Destiny 2's servers will go offline for maintenance. Bungie predicts the necessary patch to play Forsaken will go live about 30 minutes later. At 10:00 AM PT / 1:00 PM ET / 6:00 PM BST, Destiny 2 will go back online. The Forsaken expansion will be playable at that time assuming you've finished downloading the patch.