Sony will take offline PSN from 12:30 p.m. EDT to about 7 p.m. tomorrow. This maintenance session is most likely related to firmware update 4.45 which caused many consoles to freeze, rendering them completely inoperable.
PlayStation Home and Account Management will also be taken offline, however if you’re logged in now and during the maintenance times, you’ll still be able to jump into the multiplayer of your favorite game. In essence, don’t sign out of your PSN account and you’ll be fine.
To compensate, Sony’s sending the PS Plus update live one day early, which is tonight, to “allow you to hop into the store tonight ahead of tomorrow’s store update post, and pick up your purchases prior to the Tuesday downtime. Since this is an early update, be sure to take advantage of any current sales now, while you can.”
Via the US PS Blog.