@Prog! - ASP.NET(C#) AJAX -

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<asp:UpdatePanel id="UpdatePanel1" runat="server" UpdateMode="Conditional">
        <Contenttemplate>

            <asp:Button ID="Button1" runat="server" Text="Button" />
            <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>

            <cc1:PopupControlExtender ID="PopupControlExtender1" runat="server"
                TargetControlID="TextBox1"  PopupControlID="Panel1" Position="Bottom" />

            <asp:Panel ID="Panel1" runat="server" >
                <asp:Calendar ID="Calendar1" runat="server" OnSelectionChanged="Calendar1_SelectionChanged">
                </asp:Calendar>
            </asp:Panel>

        </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="Button1" EventName="click" />
        </Triggers>
</asp:UpdatePanel>