What are the animals preparing for? When do they think it will happen?

The animals are preparing for revolution.

They did not know when the Rebellion predicted by Major would take place, they had no reason for thinking that it would be within their own lifetime, but they saw clearly that it was their duty to prepare for it.

As you can see, the animals did not have a specific date in mind, but circumstances and events opened the door and the rebellion took place on Midsummer's Eve.