They have never canceled a party yet.
They have closed the park, though, due to a bad hurricane last year. IF they close the park/cancel the party because of the hurricane, I'm sure they will issue credits/refunds. That's my opinion.
BUT if they do not close or cancel anything and its just rain, then it will probably go ahead and they will decide that night whether they'll do the parade and fireworks but they'll still have the party.
Many have reported going to parties in pouring rain. It takes quite a lot to get Disney to close the park or cancel an event.
All you can do is wait and see and ask guest services the day of the party.