Assuming the user selects the days with CheckBoxes (otherwise you would have to make only minor adjustments):
List<DayOfWeek> selectedDays = new List<DayOfWeek>();
if (MondayCheckBox.Checked) selectedDays.Add(DayOfWeek.Monday);
if (TuesdayCheckBox.Checked) selectedDays.Add(DayOfWeek.Tuesday);
var onSelectedDays = DailyTimeIntervalScheduleBuilder.Create()
.OnDaysOfTheWeek(selectedDays.ToArray());
var onSelectedDays = DailyTimeIntervalScheduleBuilder.Create()
.OnDaysOfTheWeek(selectedDays);