When trying to drag and drop an appointment over the All Day slot, the ghost element is not positioned properly. This is easily observed when the browser is zoomed.

Replicated on the Overview demo

• https://demos.telerik.com/aspnet-ajax/scheduler/examples/overview/defaultcs.aspx

Hover over the command arrows on the radScheduler when the view is set to month view.  The Tooltip will display "next day" and "previous day" instead of "next month" and "previous month".

The same happens for week view.

Temporary workaround: 

<telerik:RadCodeBlock runat="server">
    <script>
        function pageLoadHandler() {
            var scheduler = $find("<%= RadScheduler1.ClientID %>");
            var $scheduler = $telerik.$(scheduler.get_element());
            var viewType = "Day";
            if (scheduler.get_selectedView() == Telerik.Web.UI.SchedulerViewType.WeekView) {
                viewType = "Week";
            } else if (scheduler.get_selectedView() == Telerik.Web.UI.SchedulerViewType.MonthView) {
                viewType = "Month";
            }

            $scheduler.find(".rsPrevDay").attr("title", "Previous " + viewType)
            $scheduler.find(".rsNextDay").attr("title", "Next " + viewType)

            // Sys.Application.remove_load(pageLoadHandler);  
        }
        Sys.Application.add_load(pageLoadHandler);
    </script>
</telerik:RadCodeBlock>

https://www.screencast.com/t/OQyCn2jM

When the timezone of the client is before UTC (e.g. UTC -1 or UTC -3) if you use the calendar for navigation, you will be navigated to the day before the selected one.

https://www.screencast.com/t/KlQtYDOxl

Workaround: 

var $ = $ || $telerik.$;
var $T = Telerik.Web.UI;
var $TS = $T.Scheduler,
$TSR = $TS.Rendering,
$DateTime = $TS.DateTime;
var $SB = Telerik.Web.StringBuilder,
timePerMinute = 60000,
timePerHour = timePerMinute * 60,
timePerDay = timePerHour * 24,
daysInWeek = 7,
roundedCornersHeight,
anchorDateFormat = "yyyy-MM-dd";

$TSVM = Telerik.Web.UI.Scheduler.Views.Month;
$TSVM.Model.prototype._createTimeSlot = function (rawIndex, startTime, element) {
    if (startTime.getHours() == 23) {
        startTime = $DateTime.add(startTime, 2 * timePerHour);
    }

    return new $TSVM.TimeSlot(rawIndex, startTime, element);
}

$TSVM.Model.prototype.getTimeSlotFromDomElement = function (domElement) {
    var rawIndex = this._getRawIndexFromDomElement(domElement),
                startTime = this._getTimeFromDomElement(domElement);
    if (startTime.getHours() == 23) {
        startTime = $DateTime.add(startTime, 2 * timePerHour);
    }

    return new $TSVM.TimeSlot(rawIndex, startTime, domElement);
}


Telerik.Web.UI.Scheduler.Rendering.MonthViewRenderer.prototype._updateColumnHeaders = function () {
    var renderer = this,
        dayHeaderDateFormat = this._model.get_dayHeaderDateFormat(),
        firstDayHeaderDateFormat = this._model.get_firstDayHeaderDateFormat(),
        currentMonth = renderer._owner.get_selectedDate().getMonth(),
        visibleRangeStart = renderer._model.get_visibleRangeStart();

    $.each(this._contentTables, function () {
        var dateLabels = $("td div.rsDateWrap div.rsDateBox a", this),
            dayStart = visibleRangeStart,
            weekDayIndex = 0,
            weekStart = new Date(dayStart.getTime());

        dateLabels.each(function () {
            var isFirstVisibleDateOfMonth =
                    $DateTime.subtract(dayStart, visibleRangeStart) == 0 &&
                    dayStart.getMonth() == currentMonth;
            var dateHeaderText;

            if (renderer._owner._renderMode == $T.RenderMode.Mobile) {
                dateHeaderText = dayStart.getDate().toString();
            }
            else if (dayStart.getDate() == 1 || isFirstVisibleDateOfMonth) {
                dateHeaderText = dayStart.localeFormat(firstDayHeaderDateFormat);
            } else {
                dateHeaderText = renderer._getFormattedDateText(dayStart, dayHeaderDateFormat);
            }

            this.href = "#" + dayStart.format(anchorDateFormat);
            this.innerHTML = dateHeaderText;
            this.title = dayStart.localeFormat(Sys.CultureInfo.CurrentCulture.dateTimeFormat.ShortDatePattern);

            var otherMonthCell = currentMonth != dayStart.getMonth(),
                dateWrapElement = this.parentNode.parentNode.parentNode;

            $(dateWrapElement).toggleClass('rsOtherMonth', otherMonthCell);

            if (weekDayIndex == renderer._weekLength - 1) {
                // Move to the next week, skipping invisible days.
                weekStart = $DateTime.add(weekStart, 7 * timePerDay);
                dayStart = new Date(weekStart.getTime());
                weekDayIndex = 0;
            } else {
                dayStart = $DateTime.add(dayStart, timePerDay);
                weekDayIndex++;
            }
            if (dayStart.getHours() == 23) {
                dayStart = $DateTime.add(dayStart, 2 * timePerHour);
            } else if (dayStart.getHours() == 1) {
                dayStart = $DateTime.add(dayStart, -timePerHour);
            }
            if (weekStart.getHours() == 23) {
                weekStart = $DateTime.add(weekStart, 2 * timePerHour);
            } else if (weekStart.getHours() == 1) {
                weekStart = $DateTime.add(dayStart, -timePerHour);
            }
            // Wrappers are cleared here to avoid second
            // traversal of the cells.
            renderer._clearExtraWraps(dateWrapElement);
        });
    });
}

The Button text initially shows "Never", even though the appointment has a different recurrence. https://www.screencast.com/t/55Rl1CSa7F7y

This doesn't work: apt.Description = DescriptionText.Text;

There is a display issue when using the agenda view - if there are fewer than 4 events displayed then clicking on the date range in the header shows a calendar which is clipped at the bottom. Presumably this is due to it being constrained by the height of the events area. This occurs even though there is plenty of height available to the control. Please see the attached screenshot which is taken from your demo site:
http://demos.telerik.com/aspnet-ajax/scheduler/examples/overview/defaultcs.aspx