It's going to depend on which Calendar control yopu ar eusing. There are two main ones:
Calendar Class (System.Windows.Controls) | Microsoft Docs
Calendar Class (System.Web.UI.WebControls) | Microsoft Docs
The first one is used in WPF apps, the second in web sites.
We have no idea what environment your app is running.
The WPFone has a Calendar.SelectedDatesChanged Event (System.Windows.Controls) | Microsoft Docs
] which tells you when the user did something, the website one doesn't, because that would be silly - given that C# code is always executed on the server, not the client, and a round trip would be very user unfriendly.
If you aren't using either of those, you would need to look at what your control provides, we have no idea what it is or where you got it!
Whatever way, start with the control documentation, and see what it provides and what sample code is available.