Hi @Josep_Ponsati,
In the case of the Eventcalendar, both [class] and [cssClass] are handled as input options internally, and they will not be applied to the top-level element, but rather to the first child element. So it depends on what you need to do. If you have to apply it to the top-level (mbsc-eventcalendar) you have to use the ngClass native functionality.
Let me know if this explains