Butter can be left out overnight safely if it is salted butter and stored properly in a covered butter dish to protect it from light, air, and contaminants. Salted butter has preservative properties that allow it to stay fresh for 1-2 days at room temperature, especially in a cool kitchen under 70°F. Unsalted butter, however, is more prone to spoilage and should ideally not be left out more than a few hours to overnight. While many people keep a stick of butter on the counter for easy spreading without issues, it is recommended to keep the butter covered and in a cool place. Butter left out too long, especially unsalted, can spoil or lose flavor and become rancid. Refrigerating butter after use is generally recommended to maintain its freshness and safety over time. In summary, leaving salted butter out overnight is generally safe if it is stored in a covered container, but unsalted butter should be refrigerated if not used within a few hours to prevent spoilage.