https://www.gamespot.com/articles/destiny-2-servers-down-for-maintenance-today-updat/1100-6454181/

Bungie has announced the next round of server maintenance for Destiny 2. The servers will be down on Thursday, October 19, for a period of four hours and will be followed by the release of a new update. [Update: Maintenance concluded sooner than expected, and a new Destiny 2 update is now available. It's primarily concerned with fixing bugs, including a resolution for Raid keys disappearing.]

A post on Bungie's website explains that the downtime will begin at 8 AM PT, at which time you won't be able to log in. At 9 AM PT, everyone who is still in an activity will be booted to the title screen. The maintenance is scheduled to end at 12 PM PT.