In Animal Farm by George Orwell, why does Major ask the animals to meet in the barn after Mr. Jones has gone to sleep?

Old Major wants to gather all the animals together in the barn so he can give them an inspiring speech. He doesn't have much longer to live and so it's a matter of urgency that he deliver the speech as soon as possible. Once Mr. Jones is fast asleep and all the animals have been gathered together, Old Major launches into his speech, which is nothing less than a revolutionary call to arms.
Under man's control, the life of an animal is short and miserable, says Old Major. Man couldn't care less about the welfare of animals; he simply sees them as an object of exploitation. He takes the fruit of their labor without actually producing anything himself. The only solution to this deeply unjust economic system is to get rid of it once and for all.
That means that the animals must rise up and overthrow the tyrannical rule of man. Old Major had a dream in which mankind had vanished and where the world was therefore a much better place. He wants to make that dream come true, and the only way to do that is by staging a rebellion against the hated human oppressor and implementing a radical Animalist revolution.