question: public DailyTimeIntervalScheduleBuilder onDaysOfTheWeek ( Set < Integer > onDaysOfWeek ) { if ( onDaysOfWeek == null || onDaysOfWeek . size ( ) == 0 ) throw new IllegalArgumentException ( "Days of week must be an non-empty set." ) ; for ( Integer day : onDaysOfWeek ) if ( ! ALL_DAYS_OF_THE_WEEK . contains ( day ) ) throw new IllegalArgumentException ( "Invalid value for day of week: " + day ) ; this . daysOfWeek = onDaysOfWeek ; return this ; }
target:  Set the trigger to fire on the given days of the week.