The answer is yes. It's happened before: on 9/11/2001 (a Tuesday), SEC Chairman Harvey Pitt did halt trading the following Wednesday through Friday, and reopened Monday, September 17.
Should they? I don't know, but it should be at least considered if there is significant damage.
This would greatly relieve the pressure on the NYSE to open Monday under what may be difficult circumstances. The NYSE has gone through great lengths to ensure a Monday open, keeping key officials in downtown hotels near the NYSE over the weekend. But with the mayor shutting down all public transportation — possibly into Monday — getting all but a skeletal crew for the floor together Monday may be impossible.
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