Hello Meppy,

I'd like to bind the Dates Property to a DateCollection, which is in another class.

This is my XAML Code so far:
<planner:Calendar Theme="{Binding Path=Theme}">
   <planner:Calendar.TimetableSettings>
      <planner:TimetableSettings Dates="{Binding
          Path=DateCollection}"> ...

Now you see that I already did that with the Theme Property of the Calendar and it works perfectly. But it won't work with Dates, although I did the Binding exactly like the Theme Binding.
I tried to debug it and it doesn't even go inside get{...}.

I saw some answers how to deal with that problem in the code-behind, but I have to work strictly with the MVVM pattern.

Thanks in advance 

Thanks for your response, but sadly it didn't help. As I already said, I did everyting just like with the Theme and there everyting worked perfectly.

I tried to debug the DataBinding with the System.Diagnostics class and this is what I get:

System.Windows.Data Warning: 56 : Created BindingExpression (hash=53954942) for Binding (hash=11903911)
System.Windows.Data Warning: 58 :   Path: 'DateCollection'
System.Windows.Data Warning: 60 : BindingExpression (hash=53954942): Default mode resolved to OneWay
System.Windows.Data Warning: 61 : BindingExpression (hash=53954942): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 62 : BindingExpression (hash=53954942): Attach to MindFusion.Scheduling.Wpf.TimetableSettings.Dates (hash=52136226)
System.Windows.Data Warning: 67 : BindingExpression (hash=53954942): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=53954942): Found data context element: TimetableSettings (hash=52136226) (OK)
System.Windows.Data Warning: 71 : BindingExpression (hash=53954942): DataContext is null
System.Windows.Data Warning: 65 : BindingExpression (hash=53954942): Resolve source deferred
System.Windows.Data Warning: 67 : BindingExpression (hash=53954942): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=53954942): Found data context element: TimetableSettings (hash=52136226) (OK)
System.Windows.Data Warning: 71 : BindingExpression (hash=53954942): DataContext is null

With the working Binding it looks like this:

System.Windows.Data Warning: 56 : Created BindingExpression (hash=3957675) for Binding (hash=29135240)
System.Windows.Data Warning: 58 :   Path: 'Theme'
System.Windows.Data Warning: 60 : BindingExpression (hash=3957675): Default mode resolved to OneWay
System.Windows.Data Warning: 61 : BindingExpression (hash=3957675): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 62 : BindingExpression (hash=3957675): Attach to MindFusion.Scheduling.Wpf.Calendar.Theme (hash=12674872)
System.Windows.Data Warning: 67 : BindingExpression (hash=3957675): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=3957675): Found data context element: Calendar (hash=12674872) (OK)
System.Windows.Data Warning: 71 : BindingExpression (hash=3957675): DataContext is null
System.Windows.Data Warning: 65 : BindingExpression (hash=3957675): Resolve source deferred
System.Windows.Data Warning: 67 : BindingExpression (hash=3957675): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=3957675): Found data context element: Calendar (hash=12674872) (OK)
System.Windows.Data Warning: 78 : BindingExpression (hash=3957675): Activate with root item MainWindowVM (hash=31874772)
System.Windows.Data Warning: 108 : BindingExpression (hash=3957675):   At level 0 - for MainWindowVM.Theme found accessor RuntimePropertyInfo(Theme)
System.Windows.Data Warning: 104 : BindingExpression (hash=3957675): Replace item at level 0 with MainWindowVM (hash=31874772), using accessor RuntimePropertyInfo(Theme)
System.Windows.Data Warning: 101 : BindingExpression (hash=3957675): GetValue at level 0 from MainWindowVM (hash=31874772) using RuntimePropertyInfo(Theme): 'Windows2003'
System.Windows.Data Warning: 80 : BindingExpression (hash=3957675): TransferValue - got raw value 'Windows2003'
System.Windows.Data Warning: 89 : BindingExpression (hash=3957675): TransferValue - using final value 'Windows2003'

I can't find any solution online 

Here's the small version of the solution.
Thanks for your help.

It works now with your assemblies. Thank you very much!