code: public NumberExpression<Integer> dayOfWeek() {
          if (dayOfWeek == null) {
              dayOfWeek = Expressions.numberOperation(Integer.class, Ops.DateTimeOps.DAY_OF_WEEK, mixin);
          }
          return dayOfWeek;
      }
docs: Create a day of week expression (range 1-7 / SUN-SAT)
  <p>NOT supported in JDOQL and not in Derby</p>
  
  @return day of week