I thought you all might be interested in this part of the blog post:
Some of you have been curious about what our engineers are up to during these maintenance periods. As you can imagine, running the system reliably requires constant nurturing. Some maintenance tasks are more fundamental to the way the system operates and are not easily done on the fly -- as a result, it's most efficient and helpful for us to take the system offline for a few hours. With that in mind, our engineers line up these tasks in advance and implement them during our regularly scheduled downtimes.
International maintenance times are also available on the blog.