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.
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.