Ops is doing a failover test between the primary and secondary data centers. You will be able to read, but not edit, during a critical point in the work. It should last about 20 to 30 minutes on these two dates:
More information (including a link to the official schedule) is at https://meta.wikimedia.org/wiki/Tech/Server_switch_2017
(User:SGrabarczuk (WMF), is this on your calendar yet?)