I have implemented it now with getting the TemplateContext including the date and checking if this date is in the DataSource and has the crossed out flag and then I set the div in the CellTemplate.
This works fine but it seems the empty cells before and after the valid cells have just random dates which leads to them also having this crossed out div as they supply valid dates in the TemplateContext although they are no dates at all. This seems to be a bug in my opinion and it makes it impossible to achieve what I'm looking for.
Do you have any suggestions on how to ignore the empty cells for the cellTemplate?