The PlayStation Network will be taken offline on Tuesday, April 21 for some essential maintenance according to Community Team Leader MusterBuster over at the Official EU PlayStation Forums. The maintenance will begin at 7:00 AM (GMT +1) during which time the entire network will be unavailable. The maintenance time frame was not mentioned so it could take several hours before the network is back online.
Apart from essential maintenance, the reason for the downtime was not mentioned. Some are speculating that the update is to fix a few bugs with the recent Firmware 2.70 update as well as preparation for the Killzone 2 patch which is due for release on Wednesday, 22nd 2009.