They do that on all servers to to back up and reboot servers to help keep them from crashing. At least that is what im told they do it for. Also they go down for around 30 min or so. Dont have an exact time cause im never on when this happens.
It's also when updates are done. The most I've seen it down for is about 30min. When they came back online I had to download a huge patch. All the MMOs I've seen also have down time.