1) First check that you have take reference of AJAX assembly.
<![CDATA[<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
2) Use Textbox control and calender extender control as below.
<asp:TextBox ID="txtValidateFrom" runat="server"></asp:TextBox>
<cc1:CalendarExtender ID="dtpDate" runat="server" TargetControlID="txtValidateFrom"
PopupPosition="BottomLeft">
3) You can also user mask edit extender and mask validator control to validate date.
<cc1:MaskedEditExtender ID="mskedtDate" TargetControlID="txtValidateFrom" Mask="99/99/9999"
MessageValidatorTip="true" runat="server" MaskType="Date" InputDirection="RightToLeft"
AcceptNegative="Left" DisplayMoney="Left" ErrorTooltipEnabled="True" />
<cc1:MaskedEditValidator ID="MaskedEditValidator5" runat="server" ControlExtender="mskedtDate"
ControlToValidate="txtValidateFrom" EmptyValueMessage="Date is required" InvalidValueMessage="Date is invalid"
Display="Dynamic" ValidationGroup="save" EmptyValueBlurredText="*" InvalidValueBlurredMessage="*" />
%>]]>