The cancellation policy for your event should be clear, concise and preferable friendly. You should include:
Dates that refunds are available.
If there is a fee to cancel and if so how much.
How to request a refund, who to contact, etc.
Ideally an alternative such as allowing registrants to transfer their registration to another person or attend a different event.
Sample 1 - this covers everything and offers a friendly alternative to paying the cancellation fee.
Any refund requests prior to [date] will be subject to a [$0.00] fee. No refunds will be issued after [date]. You may transfer your registration to another person at no cost. Please contact [name, email and/or phone number] for any cancellation and transfer requests.
Sample 2 - In some cases this is all that you need.
Sorry, there are no refunds.
Also see this article "Cancellation Policies that Benefit Event Planners and Participants".