Destiny 2 will be offline for 6 hours of maintenance today in preparation for the return of the factions Dead Orbit, Future War Cult and New Monarchy. During that window you'll be logged out and won't be able to play until it completes. So if you've not visited Xur yet then be quick.
Here are the times according to Bungie. Basically, if you're already playing when the maintenance starts you'll squeeze an extra hour out, but by 7 AM PDT 3 PM UTC you'll be signed out regardless.
Destiny 2 Update 1.0 .3: Server Maintenance and Downtime
Destiny 2 will be taken offline on Monday September 25 for server maintenance. Players will be required to download and install Destiny 2 Update 1.0.3 upon maintenance completion.
6 AM PDT (2 PM UTC): Players will no longer be able to sign in to Destiny 2.
7 AM PDT (3 PM UTC): All online players will be removed from Destiny.
12 PM PDT (10 PM UTC): Expected maintenance conclusion, Destiny 2 Update 1.0.3. available.
In return for all that not playing Destiny, Factions will return, bringing new gear and rewards including one especially powerful piece of gear.
We'll bring you more on that when it arrives but, for now, if you had anything to do in Destiny then getting done sooner rather than later.