Building on what Sunspark said, if you don't already ring external calls to a seperate button, create one and route the external calls there normally. Then on the days you don't want to take external calls, forward that button to wherever you want it. You can forward it via FACs or by setting up the system to do it while in night mode.
If using one of the night modes then you just need to be sure you're "Operator Present".
Ralph