Add this code in page for Ajax Popup Calender
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:TextBox ID="TxtFdate" runat="server" Width="142px"
ToolTip="Clict To Select Start Date" AutoPostBack="True"
ontextchanged="TxtFdate_TextChanged"></asp:TextBox>
<asp:CalendarExtender ID="TxtFdate_CalendarExtender" runat="server"
Enabled="True" TargetControlID="TxtFdate" Format="yyyy-MM-dd">