"He goes to work out every single Sunday."
The noun Sunday is modified by two adjectives every single. Every is a necessary descriptor since it conveys necessary meaning and therefore comes before the noun. Single is mere commentary and is not crucial and therefore follows the noun. That is my guess after reading numerous blogs on Chinese adjectives. Google translate agrees with Duo and translates the sentence precisely the same as Duo. Although Google is by no means an authority, they usually get the simple stuff.