code: static DateValue nextWeekStart(DateValue d, Weekday wkst) {
          DTBuilder builder = new DTBuilder(d);
          builder.day += (7 - ((7 + (Weekday.valueOf(d).javaDayNum
                  - wkst.javaDayNum)) % 7))
                  % 7;
          return builder.toDate();
      }
docs: the earliest day on or after d that falls on wkst.
  @param wkst the day of the week that the week starts on