With event planning it comes down to timing.

• Do you have the numbers to run the event?
• What is your cut off point for making the decision whether to cancel or not?
• Will you have to pay cancellation fees?
• Can you delay / re-schedule the event?
• Do you telephone, email or write to the registrants / attendees?
• Do you have speakers you need to speak to?
• What about catering? Audio Visual?

A lot of the above will depend on the size of the event and where you are holding it. But it's a good idea to check the clauses when making the initial arrangements - so if things do go pear shaped you know where you stand and what you need to do.