If you look at the FOMC announcement schedule here, you will see that since 2011, there have been 4 unscheduled meetings. By any chance, though they are "unscheduled" do market participants know one day or few hours before the meeting that there will be an actual "unscheduled" meeting or it is known to market participants only after the meeting took place?
Investors only know about the scheduled meetings. They only find out about the emergency meetings afterwards when the Fed issues a press release the next day at say 8am, which surprises the market. It would be highly improper and illegal for anyone to know about it in advance.