fd::iso_first_dow_from — Return the first day of week in ISO 8601 reference system.
// In header: <fd/date_time.h> iso_weekday iso_first_dow_from(special_iso_weekday firstDow);
Returns:
Unchanged value for ISO 6801 weekday names, determined from locale for iso_first_dow_from_system_locale, iso_first_dow_from_user_locale.
