Update: The maintenance went as planned.
tl;dr; Planned service interruption that will impact all Stack Exchange sites, Jobs, and Chat. All sites may be read-only for up to 60 minutes on Thursday, July 29, 2021 at 12:00am (midnight) UTC (Wednesday July 28 8PM US/EDT). Stack Overflow might experience some instability during this time as well. Enterprise cloud hosted instances will not be impacted.
There will be a service degradation for up to 60 minutes 12:00am UTC (midnight) on July 29th, 2021 (Wednesday July 28th 8:00 PM US/EDT). 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?
Our primary database servers, which power Stack Overflow, Stack Exchange, Chat, and other things, run out of New York (really New Jersey) with our secondary location in Colorado. When we need to perform maintenance on the primary servers, we need to failover to another server so we can patch, upgrade, and reboot them.
What we'll be doing
During the service interruption, we'll be performing two failovers from the primary location to a secondary server in New York for the SQL Servers that support the Stack Exchange Network, jobs, and chat. We're doing multiple failovers to get some detailed information of failover performance for Microsoft.
We expect that the site will be in a read-only state for less than 60 minutes in total. Once we've done the failover, and everything seems to be up and running, we'll take the sites out of read-only.
We'll be doing some behind the scenes work to hopefully minimize the impact to Stack Overflow during the failovers.
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.