It's not up for argument... McDonald's has the best fries, and tomorrow they are free. But of course there is a catch!
First off, you'll need an iPhone... Sorry Android folks! The deal is via Apple Pay, so make sure you have that setup. Then all you have to do to get your free medium order of fries, is use Apple Pay to check out in the McDonald's iOS app.
If you can't take advantage tomorrow (April 20th), they'll have the same deal the following Friday, April 27th.