set
<asp:TextBox ID="txtDate" Width="245px" runat="server" CssClass="input2" onclick="Showcalander1(event)"></asp:TextBox> <img
src="images/icon_calander.gif" id="imgdate" runat="server" onclick="Showcalander1(event)"
style="display: inline" />
and from code behing on page load set this
txtDate.Attributes.Add("readonly", "readonly");