Time ranges that are defined asholidaysin Untis mode are automatically shown as days without lessons.
In addition to the settings in the holidays' calendar (Settings | School holidays) you also can use the form view of the calendar to specify whether certain days are without lessons or are (public) holidays. This differentiation is important in some countries for teacher remuneration.
You can enter free periods for the whole school in the field "Only these periods free of lessons".
If all periods after period 5 are cancelled because of extreme heat then enter "6-8". If the first period is without lessons because of church service and the fifth period is without lessons because of a school event then enter "1,5".
Periods without lessons are displayed in the timetable with the term "Free". If you add a reason of absence, it will be displayed as well in the timetable.