code: public static DayOfWeek toDayOfWeek(final Calendar self) {
          return DayOfWeek.of(self.get(Calendar.DAY_OF_WEEK)).minus(1);
      }
docs: Converts the Calendar to a corresponding {@link java.time.DayOfWeek}. If the Calendar has a different
  time zone than the system default, the DayOfWeek will be adjusted into the default time zone.
  
  @param self a Calendar
  @return a DayOfWeek
  @since 2.5.0