Hi,
maybe you want to try simple data binding.
Assuming you have assigned an object to the AccordionItem's DataContext, which contains all the neccessary properties (i. e. EventSummaryText, CheckIsVisible...) so you can bind the AccordionItem's properties.
Now you can 'transport' that very DataContext to your HeaderTemplate by declaring the Header of the AccordionItem as follows
Header="{Binding}"
and you can do simple DataBinding in your template.
By the way:
There is no need for the Grid in your DataTemplate (if you provided the full XAML-code).
Cheers