Apply discount on checkout without coupon code?

I have a website running marketpress and I need to automatically apply 25% off the entire order without a coupon code. I can't figure out a way to do this automatically. Basically every product is 25% off, but we want to make it as simple as possible for customers to checkout and get the discount.

We are going to run the special Fri-Sun. If I have to do something manually to start and remove the discount on Sunday I'm a-ok with that as well.

Any help is greatly appreciated.