There is a gameplay error on the Hay Day game. And because of that many of player is having confusion to find mystery task info. See below is the official guide.
once you take a task, you will be able to see the details of it either; in your personal derby task log or in the task banner in the upper right side of your screen, as shown in the image of this post.
Hope you will now have more fun on Hay Day. Play online now.