Is the "routine maintenance" periodical (or on some sort of schedule)? If so, how often is it? What is the MSO maintenance timetable?
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All Stack Exchange sites go offline for a short period of time during a routine maintenance cycle. Each site shows a different message when this happens. This can also be shown when developers are working on the sites.
The image you posted was originally "Meat Means Murder", which was altered to read "Meta Means Murder", as users tend to be a bit more passionate when voting and commenting here - or at least that's how it's used. Jeff Atwood resisted the creation of a meta site at all, the image came from a blog post he wrote after finally caving in and creating one.
I believe the sites are checked on a schedule to see if maintenance is needed, but actually going off line isn't really predictable, as far as I know.
In any event, it's perfectly normal. No cause for alarm. Planned protracted outages usually follow advanced notice, via a system message bar at the top of the page, a blog post, or both. Unplanned down time that might persist is usually accompanied by a blog post, or updates from individual Twitter accounts belonging to developers.
This is usually result of a new build rolling into the system, sometimes getting a few bumps on the road.
There is a new build periodically (I think even every day) you can see the current build/revision in the bottom right part of each page:
This consists of the date released (Dec 10, 2012) and the revision number. (556)
Usually it goes smoothly but sometimes when moving to new revision there will be short downtime as a result, the developers can't always predict that and even so, I don't think we need to know in advance about short downtime.