What was ironic about the animals working on the windmill on Sundays?
Sunday is supposed to be the animals' day off. But the fact that they're forced work on this day tells you that the so-called workers' paradise of Manor Farm is a total sham. Of course, Napoleon tells the animals that their work on Sundays is "strictly voluntary," but once again we need to read the small print when it comes to his promises. Anyone who doesn't "volunteer" will have their rations reduced accordingly.
This is all part of Napoleon's strategy of control. What he's doing here is fostering a sense of unity between the animals and his regime in order to better consolidate his dictatorship. He's using classic divide and rule tactics, driving a wedge between the animals—those who work and those who don't—to make them more compliant and therefore easier to control. What is essentially forced labor is presented by Napoleon as a benevolent social program, one where animals volunteer to give up their precious Sundays to come together and work for the common good. The windmill, once constructed, will serve as a shining symbol of that united endeavor.
One of the first changes for the animals after the rebellion was the ability to have Sundays off from work. Now, with the pigs in control, they "ask" the animals to work on Sunday. It is phrased a a strictly voluntary commitment, but any animal not working on the windmill has his rations cut in half.
The work on the windmill is the main focus on the farm and it is starting to fall apart. The building of the windmill was supposed to alleviate hard labor for the animals, yet it is more of a hardship than before.
Another irony to their working on Sunday is that it is "voluntary". The animals are not forced to work on Sunday if they do not want to, but if they do not volunteer, then their food rations are cut in half. The irony being that they really have no choice in the matter, as they are punished with their food rations if they do not work.
The irony of the animals working on the windmill on Sundays is that when the animals first took over the farm and made it their own, one of their observances was a day of rest on Sundays. Now, with Napoleon running the farm as a dictator, the animals work seven days a week. They are, especially in the case of Boxer, the hard-working, devoted horse, worked to death. They have become disposable in the eyes of the rulers.