The following code assigns an OnClick event to each calendar cell by altering the way the calendar control renders the cells containing the days. When clicked, the calendar posts back the day number of the day that was clicked:
Sub ca_DayRender(sender As Object, e As DayRenderEventArgs) Dim d as CalendarDay Dim c as TableCell Dim datOrigin as Date = "1-1-2000" Dim strDayNum as String d = e.Day c = e.Cell strDayNum = CStr(DateDiff("d", datOrigin, d.date)) If d.IsOtherMonth Then c.Controls.Clear Else c.Attributes.Add("OnClick", "javascript:__doPostBack('ca','" & strDayNum & "')") End IfEnd Sub