We will be upgrading SQL Server from 2014 SP1 to 2016 RTM for all production websites on Saturday June 5th at 01:00 UTC (9:00 PM EDT on June 4th). The upgrade should be transparent on the replicas, but there will be a user-facing outage for approximately 30 seconds to 5 minutes (hopefully on the short side) while we cutover to the upgraded replica in New York for the last phase of the upgrade. We will try and sync both clusters into one site offline period to minimize user interruption.
As we test SQL 2016 for a short period after the upgrade, you may see the sites come in and out of read-only mode as we address any issues. For updates during the upgrade, follow @StackStatus on Twitter.