I haven't made a decision on the exact day or time, but I'm thinking of transferring certain parts of the site between servers. We're running out of room on a couple of drives, so I'll need to do it sometime soon. I've been just moving unimportant stuff off the drives over the past few months, but I'll need to do something a little more permanent to address it.
When I actually get around to moving the files the site will probably be down for several hours. Just thought I'd try to give you a "heads up" so you weren't wondering why the site dropped for half of a day.