Hi,
I think you should set your textboxt to read only mode....
If you are using ajax toolkit then...
See this sample code:
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<td align="left">
Admission Date:
</td>
<td align="left">
<asp:textbox id="txtAdmissionDate" runat="server" maxlength="10" readonly="true">
</asp:textbox>
<cc1:calendarextender popupbuttonid="imgDatepicker" id="CalendarExtender1" targetcontrolid="txtAdmissionDate" runat="server" filtermode="ValidChars" filtertype="Custom" validchars="1234567890/">
</cc1:calendarextender>
<asp:imagebutton runat="Server" id="imgDatepicker" imageurl="~/Calendar.png" />
Please do not forget to vote if could help...