In order to help refund multiple events, you can use the "Refund entire day" menu. This operation will ask you to confirm your choice twice and will launch a task which will cancel and fully refund all orders for the day. So whatever event of the day you select, it affects the entire day.
You do not have the choice of the type of refund, it is automatically a full refund. To use another option, you must reimburse the orders one by one or send an email to let clients refund themselves or move their reservation to another date (check this article).
When the task is completed, you will receive a confirmation email.
You can add a message that will be added in the important messages section of the cancellation email that the client will receive in order to explain the reason, especially if the client has not made the cancellation request explicitly. To do this, change the following setting in Settings then Customization and "eMail - Miscellaneous" section:
Warning - remember to delete the message after you receive the email that confirms that cancellations were done so that a later "normal" cancellation does not displays a message that is not relevant to the customer.