Update: The bug report filed to Microsoft has been removed from their feedback portal. A more simple workaround for the Material theme (and for other themes as well) is adding the missing ContentTemplateSelector="{TemplateBinding ContentTemplateSelector}" to the ContentControl in the CalendarButton template. We strongly when editing control templates and styles to copy them from the resource dictionaries in the appropriate folder in your installation (e.g. ...\Themes.Implicit\<platform>\<theme>\Themes), especially when targeting complex controls.
Also that link to the visual studio issue isn't valid anymore.
This workaround doesn't work with the new material theme.
deleted by nick.whaley
This issue occurs because of a bug in the extraction of the ControlTemplate of the ButtonStyles via Visual Studio or Blend. We've managed to isolate this issue and logged it to Microsoft here: https://connect.microsoft.com/VisualStudio/feedback/details/1929959/extracted-template-style-from-visual-studio-or-blend-is-wrong . The workaround is to: Remove all TemplateBindings to Content, ContentTemplate and ContentStringFormat in the ControlTemplates of the DayButtonStyle, MonthButtonStyle, YearButtonStyle and DecadeButtonStyle, because these bindings should not be added as they are not present in the original ControlTemplate.