Add Datepicker in Asp.net

First download Ajax toolkit
use this one
AjaxControlToolkit-Framework3.5SP1-DllOnly.zip

 from this link
http://ajaxcontroltoolkit.codeplex.com/releases/view/65800


Unzip that file

The next step is to add the Ajax Control Toolkit to the Visual Studio Toolbox. Follow these steps:

1. Launch Visual Studio and create a new ASP.NET Web Forms project or website. Open Default.aspx in the Visual Studio editor.
2. Create a new Toolbox tab by right-clicking the Toolbox and selecting Add Tab. Name the new tab Ajax Control Toolkit.
3. Right-click beneath the new tab and select the menu option Choose Items... Click the Browse button and browse to the folder where you extracted the Ajax Control Toolkit. Pick the AjaxControlToolkit.dll and click the OK button to close the Choose Toolbox Items dialog.

Now its time to create the Datepicker.....

Add a ToolkitScriptManager

Before you can use any of the Ajax Control Toolkit controls in a page, you first need to add a ToolkitScriptManager to the page. You can drag the ToolkitScriptManager from the Visual Studio Toolbox window onto the page. The ToolkitScriptManager is located in the Ajax Control Toolkit tab under the Toolbox.

1. <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">  
2. </asp:ToolkitScriptManager>  

Add a TextBox Control

The CalendarExtender works with a standard ASP.NET TextBox control. In Design view, drag a TextBox control from under the Standard tab in the Toolbox onto your page.

Next, change the ID of the TextBox control to txtStartDate. You can change the ID in the Properties window. The resulting source code looks like this:

1. <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">  
2. </asp:ToolkitScriptManager>  
3.   
4. <asp:TextBox ID="txtStartDate" runat="server"></asp:TextBox>  

Add a CalendarExtender

The next step is to apply a CalendarExtender control to the TextBox. Add the following CalendarExtender control to your page:

1. <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">  
2. </asp:ToolkitScriptManager>  
3.   
4. <asp:TextBox ID="txtStartDate" runat="server"></asp:TextBox>  
5.   
6. <asp:CalendarExtender   
7.     ID="CalendarExtender1"   
8.     TargetControlID="txtStartDate"   
9.     runat="server" />  

The following TextBox illustrates how the CalendarExtender works. Click or tab into the TextBox and the popup calendar will appear: