Why is Hump Day a Thing?

I understand that it is supposed to make Wednesday seem less dreadful, like going over the hump of the week and falling into the weekend, but why do we say this?  I really don't understand it.

Is it because we feel the need to celebrate every little thing to make our lives seem less suck? Or do we just find it amusing to say the word hump?

Whatever the reason, it always makes me think of Regina's mom on Mean Girls and now the camel from Geico's commercial.

 Now excuse me while I go make myself a Hump Day treat.