Que tal si pruebas este codigo amigo...
Simplemente guardalo como calendario.aspx
En tu Inetpub/wwwroot/calendario.aspx
Luego me cuentas
<SCRIPT runat="server">
Sub ShowDate(sender As Object, e As EventArgs)
Response.Write ("<SCRIPT language=""Javascript""><!--" & VbCrlf)
Response.write ("window.opener." & Request("campo") & ".value='")
Response.Write (cal.SelectedDate.ToString("MM/dd/yyyy") & "';" & VbCrLf)
Response.Write ("self.close();" & VbCrLf)
Response.Write ("--></SCRIP")
Response.Write ("T>")
End sub
</SCRIPT>
<html>
<head><title>Calendario de actividades</title></head>
<body>
<SCRIPT language="Javascript">
this.focus();
</SCRIPT>
<form runat="server" id="form1">
<asp:Calendar id="cal" runat="server"
enableviewstate="false"
onselectionchanged="ShowDate"
Font-Name="verdana" Font-Size="12px"
TodayDayStyle-BackColor="red"
DayHeaderStyle-BackColor="lightsteelblue"
OtherMonthDayStyle-ForeColor="gray"
NextPrevStyle-ForeColor="white"
TitleStyle-BackColor="gray"
TitleStyle-ForeColor="white"
TitleStyle-Font-Bold="True"
TitleStyle-Font-Size="15px"
SelectedDayStyle-BackColor="Navy"
SelectedDayStyle-Font-Bold="True"/>
</form>
</body></html>