@Sepideh12
Firstly, what are the current laws regarding large gatherings in that area? If there is a potential that this wedding is in contravention of those laws, you would have a strong case.
No, I don't think it's right for a host to announce something of this magnitude AFTER you booked and the 48 hour grace period has passed and then just expect you to like it or lump it. However, does the listing state anywhere that large events may be held on the property from time to time? I am not sure if the host technically needs to state this or not, but if they have, then you don't have much leeway to fight it.
Anyway, I would suggest you speak to Customer Services about this if you haven't already. Be prepared though, many people are reporting that responses are extremely slow at present.