I had 5 gigs cancelled sinced January due to Snow and Icy conditions alone this year.
I have had others thru the years cancelled the day of the event and During the event due to thunderstorms, excessive heat, etc.
I just refere to my contract, a cancelation for any reason 60days before 50% payment, within 30 days 100% payment. This is a must when Im holding dates 2 years in advance.