<asp:Label ID="lblweekoff" runat="server" Font-Size="Medium" Font-Bold="true" ForeColor="Orange">
<asp:TextBox ID="txtdate" class="form-control" autocomplete="off" CssClass="datepicker" runat="server"></asp:TextBox>

<script type="text/javascript">
 function jScript() {
        $(document).ready(function () {
         var lblweekoff = document.getElementById("lblweekoff");
            if (lblweekoff.value == "Sat,Sun") {
                $('.datepicker').datepicker({
                    minDate: 0,
                    dateFormat: "yy/mm/dd",
                    changeMonth: true,
                    changeYear: true,
                    maxDate: 2020,
                    yearRange: '2018:+' + new Date().getFullYear().toString(),
                    beforeShowDay: function (date) {
                    var day = date.getDay();
                    return [(day != 0 && day != 6)];
                    }

                });
            }
            else {
                $('.datepicker').datepicker({
                    minDate: 0,
                    dateFormat: "yy/mm/dd",
                    changeMonth: true,
                    changeYear: true,
                    maxDate: 2020,
                    yearRange: '2018:+' + new Date().getFullYear().toString()


                });
            }
   }
</script>

protected void Calendar1_DayRender(object sender, DayRenderEventArgs e)
{
    if (e.Day.Date.DayOfWeek == DayOfWeek.Sunday || e.Day.Date.DayOfWeek == DayOfWeek.Monday)
    {
        e.Day.IsSelectable = false;
        e.Cell.BackColor = System.Drawing.Color.Black;
    }

    //Same for other days
}