As far as fixing bugs they normaly wait until there are quite a number of fixes and tweaks ready and just do one patch, instead of doing a patch every day for every small issue. They also start with the big game breaking problems first and work their way down. Which at least from what I saw most of the major game breaking issues have been fixed, there are still graphical things but no biggy.
As far as what they do on downtime, if it's a crash/unexpected they are spending all their time fixing what caused it to crash to get it back up. If they took it down by themselves they are most likely updating for a patch. Now sometimes if it's a crash they will take the chance to update as well.