<asp:Calendar ID="Calendar1" runat="server" BackColor="White" BorderColor="#999999" Font-Names="Verdana" Font-Size="8pt" ForeColor="Black" OnPreRender="Calendar1_PreRender" OnSelectionChanged="Calendar1_SelectionChanged"  CellPadding="4" DayNameFormat="Shortest" Height="180px" Width="200px" OnDayRender="onDayRender" >
                                        <DayHeaderStyle BackColor="#CCCCCC" Font-Bold="True" Font-Size="7pt" />
                                        <NextPrevStyle VerticalAlign="Bottom" />
                                        <OtherMonthDayStyle ForeColor="#808080" />
                                        <SelectedDayStyle BackColor="#666666" Font-Bold="True" ForeColor="White" />
                                        <SelectorStyle BackColor="#CCCCCC" />
                                        <TitleStyle BackColor="#999999" BorderColor="Black" Font-Bold="True" />
                                        <TodayDayStyle BackColor="#CCCCCC" ForeColor="Black" />
                                        <WeekendDayStyle BackColor="#FFFFCC" />
                                    </asp:Calendar>

    
public List<DateTime> SelectedDates
    {
        get
        {
            if (ViewState["Dates"] == null)
                ViewState["Dates"] = new List<DateTime>();
            return (List<DateTime>)ViewState["Dates"];
        }
        set
        {
            ViewState["Dates"] = value;
        }
    }

 protected void Calendar1_PreRender(object sender, EventArgs e)
    {

        // Reset Selected Dates
        Calendar1.SelectedDates.Clear();
        // Select previously Selected Dates
        foreach (DateTime dt in SelectedDates)
            Calendar1.SelectedDates.Add(dt);
           
        foreach (DateTime dt in Calendar1.SelectedDates)
        {
            TextBox1.Text = dt.ToLongDateString();
        }

    }


    protected void Calendar1_SelectionChanged(object sender, EventArgs e)
    {
        //Check if selected Date is in the saved list
        // Remove the Selected Date from the saved list
        if (SelectedDates.Contains(Calendar1.SelectedDate))
            SelectedDates.Remove(Calendar1.SelectedDate);
        else
            SelectedDates.Add(Calendar1.SelectedDate);

        ViewState["Dates"] = SelectedDates;

    }



    protected void onDayRender(object sender, DayRenderEventArgs e)
    {
        if (e.Day.IsSelected == true)
        {
            SelectedDates.Add(e.Day.Date);
        }
        Session["SelectedDates"] = SelectedDates;

    }
    protected void Button2_Click(object sender, EventArgs e)
    {
        TextBox56.Text = "";
        for (int i = 0; i < SelectedDates.Count; ++i)
        {
            TextBox56.Text += SelectedDates[i];
        }
    }

    protected void Calendar1_DayRender(object sender, System.Web.UI.WebControls.DayRenderEventArgs e)
    {
        if (e.Day.Date <= DateTime.Today)
        {

            e.Day.IsSelectable = false;
        }
    }