While programmatically setting the selected date property of ASP.NET calendar control the date is visibly updated only if it is visible in the current month being displayed in the calendar, or the particular date is displayed as other month date in the current month view of the calendar.

In order to select a date in another month and refresh the view appropriately we have to set the visibledate property also.

Like

Calendar1.selectedDate="2004/05/10"
Calendar1.visibleDate="2004/05/10"