std::chrono::operator<<(std::chrono::year_month_weekday)
From cppreference.com
                    
                                        
                    < cpp | chrono | year month weekday
                    
                                                            
                    |   Defined in header  <chrono>
  | 
||
|   template< class CharT, class Traits > std::basic_ostream<CharT, Traits>&  | 
(since C++20) | |
Outputs a textual representation of ymwd into os, as if by
os << std::format(os.getloc(), STATICALLY_WIDEN<CharT>("{}/{:L}/{:L}"),
                  ymwd.year(), ymwd.month(), ymwd.weekday_indexed());
where STATICALLY_WIDEN<CharT>("{}/{:L}/{:L}") is "{}/{:L}/{:L}" if CharT is char, and L"{}/{:L}/{:L}" if CharT is wchar_t.
Return value
os
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior | 
|---|---|---|---|
| P2372R3 | C++20 | the given locale was used by default | L is needed to use the given locale
 | 
See also
|    (C++20)  | 
  stores formatted representation of the arguments in a new string  (function template)  | 
  formatting support for year_month_day (class template specialization)  |