code: public static LocalDate adjustToFirstDayOfWeek(LocalDate date, DayOfWeek firstDayOfWeek) {
          LocalDate newDate = date.with(DAY_OF_WEEK, firstDayOfWeek.getValue());
          if (newDate.isAfter(date)) {
              newDate = newDate.minusWeeks(1);
          }
  
          return newDate;
      }
docs: Adjusts the given date to a new date that marks the beginning of the week where the
  given date is located. If "Monday" is the first day of the week and the given date
  is a "Wednesday" then this method will return a date that is two days earlier than the
  given date.
  
  @param date           the date to adjust
  @param firstDayOfWeek the day of week that is considered the start of the week ("Monday" in Germany, "Sunday" in the US)
  @return the date of the first day of the week
  @see #adjustToLastDayOfWeek(LocalDate, DayOfWeek)
  @see DateControl#getFirstDayOfWeek()