On which day of the week does Mr. Whymper visit the farm?

The correct answer is that Mr. Whymper visits the farm on Monday. In "Animal Farm," Mr. Whymper is a human solicitor who serves as a kind of intermediary between the animals and the outside world. His visits are significant as they show the interaction between the farm and the human society surrounding it, and they also represent the animals' attempts to gain legitimacy and support for their revolution.

Monday is established as the day Whymper visits to discuss business and transactions regarding the farm's operations. While other days may contain important events or decisions for the farm, Monday specifically is designated for Whymper's visits, which help to illustrate the complexity of the relationships developed post-revolution and the animals' struggles with their own ideals versus the reality of their situation.