"Napoleon had commanded that once a week there should be held something called a Spontaneous Demonstration." Thus, dryly, the narrator introduces a ritual that is anything but spontaneous, as long as we understand spontaneous to mean, as it normally does, a sudden, unplanned occurrence that by its nature can't be predicted. Napoleon's "spontaneous" demonstrations are, in fact, carefully choreographed: we are told that at "the appointed time" (hardly unplanned!), the animals would gather and march around the farm in "military formation." Even the order in which the animals marched was preset:
...the pigs leading, then the horses, then the cows, then the sheep, and then the poultry. The dogs flanked the procession and at the head of all marched Napoleon's black cockerel.
Further, we learn that Boxer and Clover "always" together carried a green banner that said "Long live Comrade Napoleon!" Afterwards, animals would read poems praising Napoleon and Squealer would give his report.
Since words used ironically mean the opposite of what they normally mean, calling a highly controlled, ritualized and required event a "Spontaneous Demonstration" is ironic. We learn too that the animals like the event, as it reminds them, with another layer of irony, that they are "truly their own masters," which, of course, the very controlled nature of the event proves is untrue.
Since one of the functions of Orwell's book is to show how language can be twisted to mean the opposite of what it really means in order to confuse and wrest power from people, the "Spontaneous Demonstrations" are another example of this in action.
The spontaneous demonstrations are staged by Napoleon, therefore, the irony is that they are not spontaneous, or voluntarily. There is nothing natural about the demonstrations, they are organized to send a specific message.
The spontaneous demonstrations are a propaganda tool, used by Napoleon to convince the animals that their lives are better than they seem. The animals are miserable, there isn't enough food, they have no rights and they make sacrifice after sacrifice. So when Napoleon wants to build a school for his new piglets education, he needs to distract the other animals from the fact that they are struggling and enduring hardship.
The parades, songs and banners are designed to send a message that life on Animal Farm is great. And, some animals, especially the sheep, buy into this false message.
"There are a few diversions to keep the animals’ minds off their troubles. The pigs stage “Spontaneous Demonstrations” filled with parades and songs and poems to commemorate Napoleon’s glories."