Live service games, like Destiny 2, occasionally need to go down for maintenance. Usually these periods last for a couple hours while hotfixes are applied and data is shifted around, but Destiny 2's next expansion, Shadowkeep, evidently requires a bit more downtime before its October 1 launch. On September 30, Destiny 2 and the original Destiny will go dark for about 24 hours of maintenance leading up to Shadowkeep's launch.
There's more to Shadowkeep's launch day than the release of new Destiny 2 content, which is likely why a whole day is needed. Shadowkeep's release coincides with the New Light update, which will make the first year of Destiny 2 content and all patrol areas free-to-play for all platforms—it's possible Bungie is preparing for a large influx of new players on October 1 as a result.
Bungie also needs time to migrate PC players from Battle.net to Steam, as Destiny 2 is leaving Blizzard's service in the wake of Bungie's split with Activision. It's a free transfer that should bring over PC players' Guardians and Silver without a hitch, but Bungie is warning that certain parts of Destiny 2's previous expansion, Forsaken, will be temporarily inaccessible to PC owners of the expansion immediately following Shadowkeep's launch.
In the meantime, PC players can pre-load Destiny 2 and Shadowkeep on Steam now, and Bungie has confirmed that pre-loads for Shadowkeep will come to Xbox One and PlayStation 4 on September 30 during the downtime.
For Destiny 2 players who want to catch up with all the game has to offer ahead of Shadowkeep's release, having to take a day off right before launch may be a bummer. Still, Bungie already launched cross save last month and has been pretty transparent about its plans and goals for the next year of Destiny 2 support, which will largely revolve around battle pass-like seasons. Changes like that necessitate a good chunk of downtime—if you're still not sure of what Destiny 2: Shadowkeep has to offer, you can look over our guide to the expansion while you wait for it to launch.