Sony has announced on the PlayStation Blog that the PlayStation Network will be down from 16:00 (4PM) BST until 00:00 (Midnight) BST for scheduled maintenance.
During this time you will be unable to access PlayStation Network account management, account registration or online play. The PlayStation Store will also remain down.
Sony has recommend that if you wish to play online during that time, you login before the maintenance is due to start. You will be able yo sync trophies when the service comes back up.
Source: PlayStation Blog