If you add a new date in the German version of the Yahoo calendar, there is a selection box "Anzeigen als" (show as). One possible selection is "Kostenlos". This means "at no charge" and is complete confusing in this context. I guess the original English word is "free" which should be translated in this context as "frei" (not occupied).

Note: In other contexts "free" can be really translated as "kostenlos", for example a "free beer" is "kostenlos". But it is not good to have to many free beer before translation ;)