Spotted on the Network Status Page:

Downtime scheduled - 06/11/2009 12:00 PM London Time/ 06/11/2009 4:00 AM Pacific Daylight Time

This is a notification that UK worlds will be updated at 12:00 PM London Time on the 04/11/2009. The estimated downtime will be 2 hours.

- SOE Operations team

US Scheduled Maintenance - 06/11/2009 4:00 AM Pacific Daylight Time

This is a notification that All EQ2 Worlds will be coming down at 4:00 AM PDT on 06/11/09 for maintenance.
The estimated downtime will be 2 hours.

- SOE Operations team

Here's a handy time converter to help you plan!