Below is the screenshot of the "Free app of the Week" Section.
Since the Google started allowing the temporary sale for the top paid apps and games, I have seen a dozens of the best deals. Google added the new section in the app to highlight the weekly sale.
How to Get the Free App of the Week Section?
This feature is sever side, but you need to have the latest Play Store version. Go through the below link to download the Latest Play Store update.
Play Store Update
How to Use/Access the Free App of the Week Section?
Once you got the latest version or once you find that you got the Free app of the week section (Check in menu section or can be found by scrolling down on the Apps page in the Play Store.
Currently "Card Wars Adventure Time" featured app is available to download in free. The sale ending in four days.
Source: Android Police