Update: The maintenance went as planned.
tl;dr; Planned service interruption that will impact all Stack Overflow/Stack Exchange sites. All sites may be read-only for up to 15 minutes on Wednesday September 11, 2019 at 1:00 UTC. Enterprise cloud hosted instances will not be impacted.
There will be a service degradation for up to 15 minutes 1:00 UTC (9 PM US/Eastern) on September 11th, 2019. During that time questions and answers will still display, job listings will still work, and job ads will still display. However, the site will be "read only," i.e. people won’t be able to add/edit new job listings, apply for jobs, post, edit or vote on questions/comments/answers, reputation won't change, etc. This should minimize the disruption to the majority of casual readers. We will display a banner on the sites stating we're 'read only' for maintenance.
Longer Version of What's Taking Place?
We have two datacenters, with our main datacenter in New York (actually New Jersey) and the secondary in Colorado. When we need to perform maintenance on the database servers that power Stack Overflow, Stack Exchange, Chat, and other things, we need to failover to another location - either a local secondary (i.e. one in New York) or a remote secondary (i.e. in Colorado). The failover allows us to patch the server, and reboot them.
What we'll be doing
During the service interruption, we'll be performing a failover from the primary location to a secondary server in New York. This allows us to patch the former primary SQL Server and apply Windows updates to the server. By putting the sites in a read-only state, we reduce the chance of data loss and the entire process becomes safer.
We expect that the site will be in a read-only state for less than 15 minutes. Once we've done the failover, and everything seems to be up and running, we'll take the sites out of read-only.
Questions or concerns?
Please post a comment or answer below; I'll do my best to address any concerns between now and the maintenance window.