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<asp:Calendar ID="Calendar1" TodayDayStyle-Font-Underline="false" runat="server" SelectionMode="None" OnDayRender="Calendar1_DayRender" >

                <DayStyle Font-Underline="False" Wrap="False" />

</asp:Calendar>

    protected void Calendar1_DayRender(object sender, DayRenderEventArgs e)

    {

        if (e.Day.Date < DateTime.Now.Date)

        {

            e.Day.IsSelectable = false;

            e.Cell.ForeColor = System.Drawing.Color.Gray;

        }

    }