From wonderwallweb.com:

Xbox LIVE and Xbox.com will be down for 24 hours from 0900 UK time (1000 CET) on September 29 - but it’s not all bad news! This downtime is really important as the Xbox LIVE team will be using it to prepare the service for the arrival of the New Xbox Experience later this year!

Just to be clear, The new Xbox Experience WILL NOT come online when the downtime finishes, but the service will then be ready to receive the new service when it does eventually go live. 

I think that is midnight to midnight in the US, (I don't know which US time, since you're in multiple time zones, someone work it out). This is going to lead to speculation about the release of the new xbox experience, but that source makes it clear that it isn't due out on that day. The only thing I'm worried about is losing a day of my xbox live subscription, they'd better give us an extra day, it isn't fair if we pay for a whole month/quater/year and we get one day less.

It'll be annoying to be without live for a whole day, but at least its the first day I start university, so it'll be the first day in months I've got something to do other than play my xbox all day.

i havent been on my xbox in weeks! gta4 is so hard. but i will be playing it alot, there is so many good games coming out!
i want viva pinata trouble in paradise!

Figures it'd be down on my birthday. Oh well. I've got plenty of single player games to finish anyway.

to be without live for a whole day

oh god how horrible WHAT WILL WE DO???!

Isn't it normally only down for a few hours rather than a full day?

Theyre putting the ground works for the update before they release it. hope it comes out in october.

"oh god how horrible WHAT WILL WE DO???!"

"oh god how horrible WHAT WILL WE DO???!"

So do paying subscribers get an extra day added to their account or do they just get screwed?

"So do paying subscribers get an extra day added to their account or do they just get screwed?"

"crunchUK said:

"oh god how horrible WHAT WILL WE DO???!"

"

Oh God what ever will I do...I think I'll be just fine.

lol, now really. what do we do?

"Meowayne said:

"crunchUK said:

"oh god how horrible WHAT WILL WE DO???!"

"

....

OH MYGOD it's 10 times worse than i imagined!!"

"crunchUK said:

"Meowayne said:

"crunchUK said:

"oh god how horrible WHAT WILL WE DO???!"

"

....

OH MYGOD it's 10 times worse than i imagined!!"

What is THAT?!

So much...green.

"

don't worry guys... i have the perfect plan... if we like... STEAL a warhead, we can make MS turn XBL back on or we'll BLOW UP THE WORLD!!