This is Random Medium.
Recently Medium released a feature that lets you go back and see the top posts for a certain day. This is a site that will get you the top post from a random day (as far back as September 10, 2014, since that's when the top posts for designated days disappear). You can also choose a certain day using the box above, though that's not nearly as fun.
Since Medium doesn't have an open API for this, I used a service I made called Jam API, that lets you make an api for any site using CSS query selectors, it's pretty neat, you should definitely check it out.
If you're interested in the code that powers this site, it's open sourced on Github, just click the Octocat in the top right corner.
Follow me on Twitter @gavindinubilo, if you wanna see more things like this.