Ditto to several things Perry said, particularly under 4, use "ALL" method available, and have the policy on how you decide & when you post on your site, on flyers, in email, announced at events, etc.
Indianapolis, where I have been but am not currently on the board, used cancellation of evening events in the city schools as our main trigger because that was decided by a specific time and announced widely on tv & radio (etc.) We eventually needed more precision, as "Indianapolis Public Schools" evening events might be cancelled, but then one school would go ahead an hold an athletic event :( Unfortunately, we couldn't just name a specific school, since that school might not have events to cancel that night.
Over the years we also had to cancel a few times owners of the hall we danced at didn't want to risk liability, or require their staff to work.
Location of the hall relative to the core group of dancers and talent may tilt toward or away from cancelling events. Bloomington, IN, has not (I'm told) cancelled a regular dance since sometime in the 1970's because they have core members who live close enough to the hall to walk in (so far) any weather, and can come up with at least a handful of people who can dance (one while calling) and a musician. It can be zero outside, snowing, blowing, the scheduled band or caller might not be able to come, but if hell hasn't frozen over they're going to do at least a a couple of dances.
Kathryn
Kathryn C. Millis