Thanks to work from Becky and David at IBM in ticket #6430, we can finally remove the underscore from dijit._Calendar. The underscore was there all this time, even though _Calendar itself was a functioning standalone widget, because it did not meet dijit's policy that all widgets must be accessible. DateTextBox has always used the Calendar code as a popup, but strictly speaking, the user did not need to use the calendar to navigate dates, so dijit._Calendar was officialy a private implementation for the DateTextBox form widget.

dijit.Calendar now supports keyboard navigation and ARIA markup for assistive technologies. It has always supported high contrast mode. Now you can feel confident using this widget. The APIs are subject to Dojo's compatibility promise and a11y is supported. We'll be working on making the Calendar popup in DateTextBox accessible as well, hopefully for 1.4.