I'm currently developing a UWP app and I stumbled upon this in google, https://msdn.microsoft.com/en-us/windows/uwp/controls-and-patterns/calendar-date-picker
I'm currently using date picker in selecting dates but I thinks the calendar date picker is more suitable for this. I used date picker back then because I can't find calendar date picker in my toolbox and I believed it is not available for UWP. But referring to the link above, it should be available in my toolbox.
I really really want to use the calendar date picker.
It seems that you’re developing universal windows 8.1 app, not UWP app (windows 10). So you would not use “Calendar date picker” control in your project. If you want to use it, you would need to update your OS to windows 10, and install windows 10 SDK. More information, please refer to this document about “Get started with Windows apps” for details.