std::chrono::weekday::weekday(3)

NAME
std::chrono::weekday::weekday - std::chrono::weekday::weekday

Synopsis
   weekday() = default; (1) (since C++20)
   constexpr explicit weekday( unsigned wd ) noexcept; (2) (since C++20)
   constexpr weekday( const std::chrono::sys_days& sd ) noexcept; (3) (since C++20)
   constexpr explicit weekday( const std::chrono::local_days& ld ) (4) (since C++20)
   noexcept;
   Constructs a weekday object.
   1) Default constructor leaves the weekday value uninitialized.
   2) Constructs a weekday object holding the weekday value wd. If wd == 7, the value
   held is 0. If wd > 255, the value held is unspecified.
   3) Constructs a weekday object representing the day of the week sd corresponds to.
   This constructor defines an implicit conversion from std::chrono::sys_days to
   weekday.
   4) Constructs a weekday object representing the day of the week ld corresponds to,
   as if by weekday(std::chrono::sys_days(ld.time_since_epoch())).

Parameters
   wd - a weekday value
   sd - a system days object
   ld - a local days object

Example
// Run this code
 #include <chrono>
 #include <iomanip>
 #include <iostream>
 int main()
 {
     constexpr std::chrono::weekday friday{5}; // uses overload (2)
     static_assert(friday == std::chrono::Friday);
     for (int y{2020}; y <= 2023; ++y)
     {
         const std::chrono::year cur_year{y};
         for (int cur_month{1}; cur_month != 13; ++cur_month)
         {
             const std::chrono::year_month_day ymd{cur_year/cur_month/13};
             const std::chrono::weekday cur_weekday{std::chrono::sys_days(ymd)}; // (3)
             if (cur_weekday == friday)
                 std::cout << ymd << " is " << friday << '\n';
         }
     }
 }

Output:
 2020-03-13 is Fri
 2020-11-13 is Fri
 2021-08-13 is Fri
 2022-05-13 is Fri
 2023-01-13 is Fri
 2023-10-13 is Fri