function pageLoadHandler() {
    var spreadsheet = $find("<%= RadSpreadsheet1.ClientID %>").get_kendoWidget();
    var sheet = spreadsheet.sheetByName("Sheet1");
    spreadsheet.activeSheet(sheet);
        
    // Sys.Application.remove_load(pageLoadHandler);  
}
Sys.Application.add_load(pageLoadHandler);  

<script>
    if (kendo && kendo.spreadsheet && kendo.spreadsheet.SheetEditor) {
        kendo.spreadsheet.SheetEditor.fn.activate = function (options) {
            var viewElement = this.view.element, viewWidth, scrollerElement, scrollbarWidth;
            this._active = true;
            this._rect = options.rect;
            this._range = options.range;
            this.cellInput.position(options.rect);
            this.cellInput.resize(options.rect);
            this.cellInput.tooltip(options.tooltip);
            this.cellInput.activeCell = this.barInput.activeCell = this._range.topLeft();
            this.cellInput.activeSheet = this.barInput.activeSheet = this._range._sheet;
            if (viewElement) {
                viewWidth = viewElement.width();
                scrollerElement = viewElement.find('.' + kendo.spreadsheet.View.classNames.scroller)[0];
                scrollbarWidth = scrollerElement.offsetWidth - scrollerElement.clientWidth;
                this.cellInput.element.css('max-width', viewWidth - scrollbarWidth - this.cellInput.element.position().left + 'px');
            }
            this.trigger('activate');
            return this;
        }
    }
</script>

<telerik:RadSpreadsheet runat="server" ID="RadSpreadsheet1" />
<telerik:RadSpreadsheet runat="server" ID="RadSpreadsheet2" />

protected void Page_Load(object sender, EventArgs e)
{
    var workbook = new Workbook();
    var sheet1 = workbook.AddSheet();

    AddDateRow(sheet1);

    sheet1.Columns = new List<Column>() { new Column() { Index = 1, Width = 150 } };

    RadSpreadsheet1.Sheets.Add(sheet1);
    RadSpreadsheet2.Sheets.Add(sheet1);

    RadSpreadsheet1.ColumnsCount = 5;
    RadSpreadsheet1.RowsCount = 10;
}

private void AddDateRow(Worksheet sheet1)
{
    var row = new Row() { Index = 0 };
    Cell firstCell = new Cell() { Index = 0, Value = "Select Date:", Bold = true };
    row.AddCell(firstCell);

    Cell secondCell = new Cell() { Index = 1, Value = "", Bold = true, Background = "#fef0cd", Format = "mmmm d, yyyy" };

    secondCell.Validation = new Validation()
    {
        AllowNulls = true,
        DataType = "date",
        ShowButton = true,
        ComparerType = "between",
        From = "DATEVALUE(\"1/1/2000\")",
        To = "DATEVALUE(\"12/31/2020\")",
        Type = "reject",
        TitleTemplate = "Invalid date selected",
        MessageTemplate = "Select a date between year 2000 and 2020."
    };

    row.AddCell(secondCell);
    sheet1.AddRow(row);
}

<script>
    // Place/load this script after the ScriptManager
    if (Telerik.Web.UI.RadSpreadsheet) {
        Telerik.Web.UI.RadSpreadsheet.prototype.original_initialize = Telerik.Web.UI.RadSpreadsheet.prototype.initialize;
        Telerik.Web.UI.RadSpreadsheet.prototype.initialize = function () {
            this.original_initialize();
            this.add_parentShown(this.get_element());
        }
        Telerik.Web.UI.RadSpreadsheet.prototype.repaint = function () {
            this.get_kendoWidget().refresh()
        }
    }
</script>

<telerik:RadWizard runat="server" ID="RadWizard1">
    <WizardSteps>
        <telerik:RadWizardStep ID="WizardStep1" StepType="Start">
            Step 1
        </telerik:RadWizardStep>
        <telerik:RadWizardStep ID="WizardStep3">
            <telerik:RadSpreadsheet runat="server" ID="RadSpreadsheet1"></telerik:RadSpreadsheet>
        </telerik:RadWizardStep>
        <telerik:RadWizardStep ID="WizardStep4" StepType="Finish">
            Finish step
        </telerik:RadWizardStep>
        <telerik:RadWizardStep ID="WizardStep5" StepType="Complete">
            Completed!
        </telerik:RadWizardStep>
    </WizardSteps>
</telerik:RadWizard>

<telerik:RadSpreadsheet runat="server" ID="RadSpreadsheet1" Height="2000px" Width="100%">
</telerik:RadSpreadsheet>

<script>
        kendo.spreadsheet.registerEditor('_validation_list', function () {
            var context, list;
            function create() {
                var $list = $("[id$='SpreadsheetListBoxDropDown']").first();
                var element = $list.closest(".rssPopup");

                list = $find($list.attr("id"));
                if (!this._listdropdown) {
                    var dropdown = new kendo.spreadsheet.DropDown(element);
                    this._listdropdown = dropdown;

                    list.add_selectedIndexChanged(function (sender, args) {
                        dropdown.close();
                        var item = list.get_selectedItem();
                        if (item) {
                            context.callback(item.get_value());
                        }
                    });
                }

                this._listdropdown.openFor(context.view.element.find('.k-spreadsheet-editor-button'));
            }
            function open() {
                create();
                var matrix = context.validation.from.value;
                if (matrix) {

                    var items = list.get_items();
                    items.clear();
                    matrix.each(function (el) {
                        var item = new Telerik.Web.UI.RadListBoxItem();
                        item.set_text(el);
                        items.add(item);
                    });
                }
            }
            return {
                edit: function (options) {
                    context = options;
                    open();
                },
                icon: 'p-icon p-i-arrow-60-down'
            };
        });
    </script>

protected void Page_Init(object sender, EventArgs e)
{
    var path = Server.MapPath("~/App_Data/spreadsheet.xlsx");
    var workbook = Workbook.Load(path);
    HiddenField1.Value = workbook.ToJson();
}

<script>    
    function pageLoadHandler() {
        var spreadsheet = $find("<%= RadSpreadsheet1.ClientID %>");
        var hiddenfield = $get("<%= HiddenField1.ClientID %>");
        if (hiddenfield.value) {
            spreadsheet.get_kendoWidget().fromJSON(JSON.parse(hiddenfield.value))
            hiddenfield.value = '';
        }

        // Sys.Application.remove_load(pageLoadHandler);  
    }
    Sys.Application.add_load(pageLoadHandler);
</script>
<telerik:RadSpreadsheet runat="server" ID="RadSpreadsheet1"></telerik:RadSpreadsheet>
<asp:HiddenField runat="server" ID="HiddenField1" />