Starting at 12am PT (3am ET), PSN will be down for around two hours in North America. It looks like Europe won't be affected this time.
The routine maintenance will go down basically the same as last week's maintenance
– if you signed into PSN in the last few days, you should have no problem accessing apps and playing games during the downtime; however, if you haven't logged in recently, you'll be shut out. Relax, you still have some time to log in!
The PlayStation Blog
says PSN should be back up and running around 2am PT (5am ET). During the maintenance, both the PSN Store and Account Management will be inaccessible.