Don't go there.  Don't even try it.  As much as it would rock for data entry based on a calendar date.  The problem is, that the OnDayRender event of the calendar fires too late for the stuff that your modal popup needs to do to register itself.  At least, that's my current theory, since nobody replied to my whining on the ASP.NET forums about it.  So it winds up telling you "Page cannot be null.  Make sure your modal popup is running in the context of an ASP.NET request".  In fact, the OnDayRender event is so limiting I'm sure you'll wind up scrapping the whole calendar.  Try Peter's Date Package instead.