> When's a cheap day to fly to and return from Las Vegas?

Hopper, an app on iOS, has exactly this. You give it a date, and then it gives you a month view with each day color-coded according to how expensive it will be to travel.

It also gives you recommendations about whether to buy a ticket now, or wait until prices go down.

I've bought tickets a few times through Hopper and it worked well. I have no business relationship with them at all--just a happy customer.