Add a multiple date selection to the Calendar. A multiple date selection calendar is very important for professional web applications in many situations. Also, there is a lack of events in the API.
It is always important to provide more events then less.

Here a few suggestions:
Property year;    /// Year displayed by control.
Property month; /// Month (1 to 12) displayed by control.
A method in the API to easily get the selected date(s).
A method in the API to easily set the selected date(s).
An option to indicate if we allow/disallow multi-selection.
A callback event (and passing the data as an argument) when a date is selected/unselected to allow/disallow the selection/deselection (by returning a true/false). This would allow to easy show a warning message when some dates are selected/unselected. We may currently do part of it in the change event but adding events onbefore/onafter selection would be nice.
Add the option to hide/show days from previous/next month.
Add the option to enabled/disable click from previous/next month.
The Year selection on the top should be a combo that allow keyboard input.
You may check the Yahoo Calendar event & properties. It will provide a good examples (http://developer.yahoo.com/yui/docs/YAHOO.widget.Calendar.html)