In the US, Wednesday is informally known as hump day. Why? Well, let's think about a hump. A hump is a curved, raised area on something. Look at the picture of the camels. The two lumps on their back...we call those humps.
Now that we know what a hump is, let's talk about why we call Wednesday hump day. Think about the work week (Monday-Friday) as being a hump, or a hill. First you have to climb up it. Then you go down the other side. Since Wednesday is in the middle of the work week, you are at the top of the hump. And that is why we call Wednesday hump day!