Destiny 2's servers will be taken down today as part of a now weekly update.
Starting right now at 8 AM PT/11 AM ET, you won't be able to log into the game on PS4 and Xbox One. At 9 AM PT/12 PM ET, all players will be booted from the game. The maintenance will last until 12 PM PT/3 PM ET.
The maintenance will implement hotfix version 126.96.36.199 which will mostly fix various bugs including: missed milestones for completing the Raid. Considering how difficult the Leviathan Raid is shaping up to be, it's pretty disheartening to not see the Milestone after several hours of playtime. Although Bungie told players that not seeing the Milestone didn't impact their rewards.
You can check out the full patch notes over at the Bungie blog.
Bungie also announced that server queues could be waiting for players once the maintenance period ends, which should be expected considering the high volume of players trying to log in after the maintenance period.
Destiny 2 has been pretty regular with its maintenance downtimes, so this isn't unusual. There is a chance the updates could take longer than planned so be sure to check here for any possible updates.