In this article I will explain with example how to set specific Date Format such as dd/MM/yyyy for the ASP.Net AJAX Control Toolkit CalendarExtender control.
HTML Markup
First we need to register the AJAX Control Toolkit on the page
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
Below is the HTML Markup which consists of a TextBox, an ImageButton and an ASP.Net AJAX CalendarExtender control.
<cc1:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
<asp:TextBox ID="txtDate" runat="server" ReadOnly="true"></asp:TextBox>
<asp:ImageButton ID="imgPopup" ImageUrl="images/calendar.png" ImageAlign="Bottom"
    runat="server" />
<cc1:CalendarExtender ID="Calendar1" PopupButtonID="imgPopup" runat="server" TargetControlID="txtDate"
Set dd/MM/yyyy Date format for ASP.Net AJAX CalendarExtender Popup
If you want to set any specific Date Format form example dd/MM/yyyy then simply set the required date format in the Format property of the ASP.Net AJAX CalendarExtender control.
ASP.Net AJAX CalendarExtender with ImageButton Example | ASP.Net AJAX CalendarExtender dd/MM/yyyy Date Format Example