<asp:TextBox ID="TextBox3" runat="server" ReadOnly="True" ></asp:TextBox>
 <asp:CalendarExtender ID="CalendarExtender2" TargetControlID="TextBox3"
                            runat="server">

I have use text box and calender

Instead of using the ReadOnly="True" property ... how about using the Enabled="False" property?

Another option would be to fire a javascript to set the ReadOnly property to "False" after the validation of the form, but before actually submitting the form.

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