Sony has announced that they will be scheduling a maintenance for the PlayStation Network on the 21st of April, from 9:50 AM to 3:50 PM Pacific Time.
Sony has mentioned that "If you've signed in to the network any time since April 16th you will be able to play games online and access partner apps such as Netflix during this maintenance period. The PlayStation Store, PlayStation Home and Account Management will not be available during this time, and users may receive a maintenance notification when attempting to access these services."
There may be some downtime after the scheduled time, which is dedicated to test the network before it goes live.
Source: PlayStation Forums