https://www.gamespot.com/articles/heres-when-you-can-play-destiny-2s-halloween-event/1100-6462509/

Bungie is rolling out Update 2.0.4 for Destiny 2 soon, alongside its seasonal events and a new Iron Banner. In an update, the studio has outlined its release plans so you can schedule your playtime down to the minute. Almost. Eyes up, Guardian.

On Tuesday, October 16 the event will begin. First at 7 AM PT / 10 AM ET, Destiny 2 maintenance will begin, but it won't be accompanied by downtime. Companion features might be unavailable, but at this point you should be ready to download the update. At 9:45 AM PT / 12:45 PM ET, anyone who hasn't installed 2.0.4 will be kicked out and taken back to the title screen to prompt them to install it. Then at 10 AM PT / 1 PM ET, the maintenance will be over and companion features will be reenabled.