Tomorrow morning at around 10:30ish, the site will go down for approximately 20-30 minutes.
Without going into nerdy detail, we're going to be re-tasking one of the databases.
After it's done, you won't experience any difference in performance from the site, and there isn't going to be anything that's changed with the appearance or behaviour of the site.
This is purely a utility upgrade, which will be transparent to all visitors, but it's a necessary one.