Actually the community leaders can request everyone to take off time for a focused day of prayer when their is a crises. Mordechai did that in Shushan and requested 3 days in a row. However, none of the 3 days were normal work days. They were erev Pesach, first day of Pesach and second day of Pesach (at least chol HaMoed, perhaps Shabbos).
Back to topic at hand: If anyone still alive from when we knew what tcheiles was come back and testifies that such and such is indeed tcheiles we should be able to accept him as an ordinary witness. I am not aware of a need for 2 witnesses in such matters.