Per Sony's Network Service Status page, PSN will undergo routine maintenance next Tuesday, May 24. Only PS4 will be affected.
PSN will go down beginning at 9:30p Pacific and should be back up by 11:30. During maintenance, users will be unable to access account management, the PlayStation Store, or PlayStation Video. Users should still be able to play games and "most" other applications without interrupt.
In a statement posted to Network Service Status, Sony assured users that it is working to reduce downtime for each subsequent maintenance period. Sony has also been rolling out updates to PSN's architecture, such as a planned move to two-step authentication for tighter security.