Day v. Micou is a United States Supreme Court case that was decided in 1873. The case involved a dispute between two parties over a contract for the sale of a steamboat. The plaintiff, Day, had contracted with the defendant, Micou, to purchase a steamboat for $2,000. Day had paid $1,000 of the purchase price and was to pay the remaining $1,000 upon delivery of the boat. However, when the boat was delivered, Day refused to pay the remaining $1,000, claiming that the boat was not in the condition that had been promised. The Supreme Court held that Day was not liable for the remaining $1,000. The Court reasoned that Day had a right to refuse to pay the remaining $1,000 because the boat was not in the condition that had been promised. The Court also held that Day was not liable for any damages that may have been caused by his refusal to pay the remaining $1,000. The Court reasoned that Day had acted in good faith and had not acted with any malicious intent. In conclusion, the Supreme Court held that Day was not liable for the remaining $1,000 of the purchase price of the steamboat. The Court also held that Day was not liable for any damages that may have been caused by his refusal to pay the remaining $1,000.

Justice Field delivered the dissenting opinion in Day v. Micou, arguing that a contract between two parties should not be voided due to an alleged mistake of one party if it is clear from the language of the contract that both parties intended for it to be binding. In this case, Day had agreed to sell certain property belonging to him and his wife at a public auction with Micou acting as their agent. The sale was made without any reservation or condition being expressed by either party and yet afterwards Day sought to have it annulled on grounds of mutual mistake regarding its terms. Justice Field argued that such mistakes are only valid when they relate directly back to some term or condition which was expressly stated in the agreement itself; otherwise there would be no end put upon litigation over contracts since every party could claim after-the-fact misgivings about what they had agreed upon originally. He concluded by stating that courts must respect agreements entered into freely and voluntarily between competent persons unless there is evidence proving fraud or misrepresentation on behalf of one side, neither of which were present here according to his view.