how about this trivial thing ?

   enum class days {
           MONDAY = 1,
           TUESDAY = 2,
           ...
   };

   constexpr const char* str_day(days d) {
           switch (d) {
                   case days::MONDAY: return "MONDAY";
                   case days::TUESDAY: return "TUESDAY";
                   ...
           }
   }

maybe just throw in a 'noexcept' for good measure as well.