If You uses vs 2010,Ajax controls are inbuilt.
else download ajaxcontroltoolkit dll.
register this dll in aspx page
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajaxToolkit"%>
take a text box on form in which you want to take date.
pass TargetControlID=ID of TextBox
<ajaxtoolkit:calendarextender id="cal1" behaviorid="bcal1" xmlns:ajaxtoolkit="#unknown">
TargetControlID="txtDatefrom" runat="server"></ajaxtoolkit:calendarextender>