When one party refuses to perform their duty before the performance date, what is this known as?

The situation described, where one party indicates they will not fulfill their obligations under a contract before the specified performance date, is known as anticipatory repudiation. This concept recognizes that when one party makes it clear that they will not perform their contractual duties, the other party is justified in treating the contract as breached.

Anticipatory repudiation allows the non-breaching party to seek remedies even before the performance is due, as the refusal to perform effectively undermines the entire contract. This principle is designed to protect the reliance interests of the involved parties, ensuring that one party's refusal to act does not leave the other in a position of uncertainty without the ability to seek redress promptly.

The other options are not applicable in this context. Tortious interference pertains to wrongful interference with contract relationships, a unilateral contract involves a promise made by one party without a corresponding promise from another, and an error in performance typically refers to mistakes that occur during the execution of a contract rather than a refusal to perform ahead of time.