https://www.gamespot.com/articles/destiny-2-servers-going-offline-today-heres-when-a/1100-6453518/

Bungie will conduct another round of Destiny 2 maintenance next week, which means the game will be offline for a period of time. In its latest weekly blog post, Bungie said the server maintenance period for Update 1.0.3. [Update: The Destiny 2 maintenance period is scheduled to begin this morning and will run until 12 PM PT.]

Starting at 6 AM PT on September 25, players won't be able to sign in. At 7 AM PT that day, everyone who is logged in to Destiny 2 will be kicked off. The maintenance period is scheduled to run until 12 PM PT, so all told, there will be interruptions for around six hours. That's if everything goes to plan; there is always a chance the maintenance period could wrap up sooner or, as was the case last time, take longer.