The question in Hochster is whether De La Tour's repudiating statement on May 11 is a material breach of the contract, a breach that occurs on May 11. The court held that it was.
This means that--at the time of the repudiating statement on May 11--Hochster is excused from his contractual obligation to De La Tour and is hence free to take another tour guide job with someone else. He does not have to wait until June 1 to see whether De La Tour changes his mind again and wants him as a tour guide.