You can download an App called "App of the day" in Android and IOS which will feature one premium app each day. Although you can't choose a particular premium app for free, this app does select and show you the one premium app for free to download. How they do it is that they usually approach or contact the app owner or developer and entice them into offering their app for free just for a day. This way both the party benefits because the app owner has wider audience to download the app and the "App of the day" guys has something to offer the people each day. Sometimes, they would simply display and app which are always free to download and may not be that premium app.