From MSDN forums user Magnus (MM8) (

Right-click on an element, for example a Calendar, in design mode in Visual Studio 2012 or later and select the “Edit template” option and then the “Edit a copy…” option. This will bring up a dialog that lets you specify the name and location of a style resource that is created for you when you click the “OK” button in the dialog.

There is also an “Edit Additional Templates” option and under this one you will find an “Edit CalendarDayButtonStyle” option that will copy the default template of the CalendarDayButton into your XAML markup.