So we have dates for this now. The NA data center will come down on the 15th of May and come back up in its new undisclosed location on the 17th. Midnight PDT both days. Active subscriptions (including free time from the callback campaign) will be extended 1 day for everyone. The JP data centers are also coming down for a day starting on the 16th.
So:
Downtime from 5-15 00:00 to 5-17 00:00.
SE isn't telling us where the data center is being moved to, but it's going to be a tough secret to keep as soon as it comes up. I'm betting and hoping on somewhere in the midwest. Either Chicago or Texas most likely as those tend to be popular spots for large data centers in the US.