Not without a lot of custom programming. The other controls are validated at the client and result in a message box being shown *before* the page posts back. The Calendar is validated at the server after a postback and doesn't result in the ValidationSummary displaying a message.
If you want to group all errors, you have many options, including:
1. Don't use client side validation and display all error messages as HTML after a postback using the ShowSummary setting of the Validation Summary
2. Use the above, but implement client script that alerts the contents of the validation summary after a postback.
Alternatively, use a different Calendar (from the AJAX toolkit for example: http://wiki.asp.net/page.aspx/417/calendar-extender/
or from jQuery UI: http://jqueryui.com/demos/datepicker/
) that works with client side validation as well. Then you can write a client function for the CustomValidator control to inject the error message in the alert box.