One piece of evidence does not mean that something is definitely the case or is going to happen. A reference to the migration of swallows that happens in the spring or early summer. A lot of things went right for us this year, but one swallow does not a summer make—we still have a long ways to go before our finances are back in order. A: "I'm really worried the boss is going to fire me. He said he really wasn't happy with my work lately!" B: "Calm down, one swallow does not a summer make."