From midnight Pacific Time on Tuesday October 17 for up to 24 hours, or from around 8.00am or 9.00am Tuesday morning our time, depending on where in Europe you're hiding, Microsoft will flick the 'off' switch for XBL for up to 24 hours.
Why? "Because we're adding some cool stuff. And we have to take the service and xbox.com offline in order to do it," says Microsoft although it won't tell us what, apart from the fact that it's not a Dashboard update.
The planned downtime should last no longer than 24 hours, all going well...