PSN will be undergoing a period of maintenance on the 23rd of May, Sony announced via their Network Service Status page.
The times for this will be approximately 1 hour 6:00 AM to 7:00 AM in your local time zone on the 23rd of May. This will affect the PlayStation 4, PlayStation 3, and PlayStation Vita platforms.
This will affect the ability to access certain PSN products, such as Account Management, Gaming And Social, PlayStation Store, PlayStation Music, and PlayStation Video.
Most applications and the ability to sign in and play games should remain unaffected.