Often, web applications are updated. If the update is properly planned, the maintenance is announced and visitors know what to expect and why the downtime is happening.
However, I have problems understanding the length of some of these maintenance windows. I have maintained medium-sized web applications. Since updates were tested in advance in a staging environment, updating the application involved only pushing the new code to the server, performing data migrations and reloading/restarting the server configuration, topping at 15 minutes for maintenance updates, but generally only a few seconds.
Maintenance updates at large corporations tend to take a lot longer. I have seen organizations taking 3 hours to rollout minor updates; updates to governmental systems may take eight to nine hours, or if you're unlucky an entire weekend. It is unclear to me what their process might be. I understand they may have large clusters and databases to work with, but I can't imagine what may take those amounts of time. What are they doing in those hours? Is this their inefficiency and planning for disaster, or is there something I'm missing completely?
(I'm ignoring hardware maintenance or major version rollouts here.)