Free days are not hidden because one can not guarantee that there is no burndown in practice on such days. Instead Kunagi adjusts the ideal burndown line and the future projection line.

For now it is only possible to mark specific weekdays as "free". This feature is not available for specific dates. But I could imagine this in future versions.

I'd like to point out that such a feature will propably not be in Kunagi before sto175 is done.