"Blackout days" refers to specific days or periods when certain activities or services are unavailable or restricted. It means that you cannot access or use something on those particular days. Full definition
You have to plan around the several blackout days and availability. (takeoffwithmiles.com)
In the weeks leading up, replace those Improving - Non Stop signs systemwide with «Golden Blackout Day: Sunday May 7. (secondavenuesagas.com)
The folks there have learned Apple has said April 24 is a vacation blackout day for all employeespresumably because they need all hands to deal with the onslaught of customers breaking down the doors to get an iPad. (canadianbusiness.com)