The following figure shows the school-year calendar for a lesson that has a time limitation of 05.03 to 28.03. 14.03 is a public holiday (marked red). You can call the school year calendar from any lessons view.

Despite the public holiday there is still the possibility that all the lesson periods could be held as they could be scheduled for other days.
This can be seen from the entry “Duration (Eff. time range)” on the “General data” tab. This shows the number of weeks in the time period in which at least one day of lessons can be held. In this case 4, in spite of the public holiday.
With holidays
If there were a week’s holiday within the time limitation period, e.g. from 12.03 to 16.03, this possibility would no longer exist. In this case the lesson could only take place three times within the time limitation period.

You can determine for yourself whether or not holiday weeks should influence value calculation by using the “ Count only school days ” option on the “Value Calculation” tab under “Settings | Miscellaneous”
The information on the tabs “Value Calculation” and “Values for ‘Count only school days’” in the school year calendar is useful for understanding the individual calculations.