The PlayStation Network maintenance that was originally scheduled to happen this Monday will take place tomorrow on the 28th of August. This maintenance was delayed due to the DDoS attack on Sony's PlayStation services.
The PlayStation services will be offline in Europe from tomorrow at 4pm until Friday at 5am BST, and in North America from 9:40am to 4:50pm EDT. During this time, users won't be able to log-in to Account Management, PlayStation Store, or PlayStation Home.
There are two solutions for those that want to be online during the maintenance time. Users can either sign-in with their PS4 before the maintenance starts or within the last 5 days with any other PlayStation device.
Source: PlayStation Community