When pigs fly is an informal way to joke that you will never do something. Example: "Do you think you will ever work at that company again?" Reply: "When pigs fly!"

Pigs are heavy animals that stay close to the ground and do not fly, so when pigs fly is a time that will never come. Example: "Do you think your brother will be successful?" Reply: "When pigs fly he will."

When pigs fly is a time that will never come because pigs will never fly. Example: "Will you ever take her on a date?" Reply: "Sure: When pigs fly!"