@inject IJSRuntime _jsInterop
@* this errors suppression is a hack to make this sample succinct
    move this script to a proper place for a real app*@
<script suppress-error="BL9992">
    function clickSchedulerPromptButton(btnIndex) {
        setTimeout(function () {
            var buttons = document.querySelectorAll(".k-window-content .text-right button");
            if (buttons && buttons.length >= btnIndex) {
                var chosenButton = buttons[btnIndex];
                chosenButton.click();
            }
        }, 50);
    }
</script>

@* appearance setting for the template - make the custom template tall as the appointment to capture all clicks *@
<style>
    .tallAppt {
        height: 100%;
    }
</style>

<TelerikScheduler Data="@Appointments"
                  OnUpdate="@UpdateAppointment"
                  OnCreate="@AddAppointment"
                  OnDelete="@DeleteAppointment"
                  AllowCreate="true" AllowDelete="true" AllowUpdate="true"
                  @bind-Date="@StartDate" Height="600px" @bind-View="@CurrView">
    <ItemTemplate>
        @{
            SchedulerAppointment appt = context as SchedulerAppointment;
        }
        <div title="@appt.Start - @appt.End" class="tallAppt" @ondblclick="@( () => ChooseEditMode(appt) )"><div class="k-event-template">@appt.Title</div></div>
    </ItemTemplate>
    <AllDayItemTemplate>
        @{
            SchedulerAppointment appt = context as SchedulerAppointment;
        }
        <div title="@appt.Start.ToShortDateString() - @appt.Title" class="tallAppt" @ondblclick="@( () => ChooseEditMode(appt) )"><div class="k-event-template">@appt.Title</div></div>
    </AllDayItemTemplate>
    <SchedulerViews>
        <SchedulerDayView StartTime="@DayStart" />
        <SchedulerWeekView StartTime="@DayStart" />
        <SchedulerMultiDayView StartTime="@DayStart" NumberOfDays="10" />
    </SchedulerViews>
</TelerikScheduler>

@code {
    //async void so we don't block the execution
    //we will have a small timeout in the script to let it wait for the popup
    async void ChooseEditMode(SchedulerAppointment appt)
    {
        // check if we have a recurring appointment or a member of one
        if (appt.RecurrenceId != null || !string.IsNullOrEmpty(appt.RecurrenceRule))
        {
            int btnIndexToClick = 0;//the first button - edit instance
                                    // make it 1 for the second button - the series

            await _jsInterop.InvokeVoidAsync("clickSchedulerPromptButton", btnIndexToClick);
        }

    }

    //the rest is sample data and sample CUD operations handling



    // sample data and scheduler settings
    public SchedulerView CurrView { get; set; } = SchedulerView.Week;
    public DateTime StartDate { get; set; } = new DateTime(2019, 12, 2);
    public DateTime DayStart { get; set; } = new DateTime(2000, 1, 1, 8, 0, 0); //the time portion is important

    List<SchedulerAppointment> Appointments { get; set; }

    async Task UpdateAppointment(SchedulerUpdateEventArgs args)
    {
        SchedulerAppointment item = (SchedulerAppointment)args.Item;

        await MyService.Update(item);
        await GetSchedulerData();
    }

    async Task AddAppointment(SchedulerCreateEventArgs args)
    {
        SchedulerAppointment item = args.Item as SchedulerAppointment;

        await MyService.Create(item);
        await GetSchedulerData();
    }

    async Task DeleteAppointment(SchedulerDeleteEventArgs args)
    {
        SchedulerAppointment item = (SchedulerAppointment)args.Item;

        await MyService.Delete(item);
        await GetSchedulerData();
    }






    public class SchedulerAppointment
    {
        public Guid Id { get; set; }
        public string Title { get; set; }
        public string Description { get; set; }
        public DateTime Start { get; set; }
        public DateTime End { get; set; }
        public bool IsAllDay { get; set; }
        public string RecurrenceRule { get; set; }
        public List<DateTime> RecurrenceExceptions { get; set; }
        public Guid? RecurrenceId { get; set; }

        public SchedulerAppointment()
        {
            Id = Guid.NewGuid();
        }
    }

    async Task GetSchedulerData()
    {
        Appointments = await MyService.Read();
    }

    protected override async Task OnInitializedAsync()
    {
        await GetSchedulerData();
    }

    // the following static class mimics an actual data service that handles the actual data source
    // replace it with your actual service through the DI, this only mimics how the API can look like and works for this standalone page
    public static class MyService
    {
        private static List<SchedulerAppointment> _data { get; set; } = new List<SchedulerAppointment>()
{
            new SchedulerAppointment
            {
                Title = "Board meeting",
                Description = "Q4 is coming to a close, review the details.",
                Start = new DateTime(2019, 12, 5, 10, 00, 0),
                End = new DateTime(2019, 12, 5, 11, 30, 0)
            },

            new SchedulerAppointment
            {
                Title = "Vet visit",
                Description = "The cat needs vaccinations and her teeth checked.",
                Start = new DateTime(2019, 12, 2, 11, 30, 0),
                End = new DateTime(2019, 12, 2, 12, 0, 0)
            },

            new SchedulerAppointment
            {
                Title = "Planning meeting",
                Description = "Kick off the new project.",
                Start = new DateTime(2019, 12, 6, 9, 30, 0),
                End = new DateTime(2019, 12, 6, 12, 45, 0)
            },

            new SchedulerAppointment
            {
                Title = "Trip to Hawaii",
                Description = "An unforgettable holiday!",
                IsAllDay = true,
                Start = new DateTime(2019, 11, 27),
                End = new DateTime(2019, 12, 05)
            },

            new SchedulerAppointment
            {
                Title = "Morning run",
                Description = "Some time to clear the head and exercise.",
                Start = new DateTime(2019, 11, 27, 9, 0, 0),
                End = new DateTime(2019, 11, 27, 9, 30, 0),
                RecurrenceRule = "FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR"
            }
        };

        public static async Task Create(SchedulerAppointment itemToInsert)
        {
            itemToInsert.Id = Guid.NewGuid();
            _data.Insert(0, itemToInsert);
        }

        public static async Task<List<SchedulerAppointment>> Read()
        {
            return await Task.FromResult(_data);
        }

        public static async Task Update(SchedulerAppointment itemToUpdate)
        {
            var index = _data.FindIndex(i => i.Id == itemToUpdate.Id);
            if (index != -1)
            {
                _data[index] = itemToUpdate;
            }
        }

        public static async Task Delete(SchedulerAppointment itemToDelete)
        {
            if (itemToDelete.RecurrenceId != null)
            {
                // a recurrence exception was deleted, you may want to update
                // the rest of the data source - find an item where theItem.Id == itemToDelete.RecurrenceId
                // and remove the current exception date from the list of its RecurrenceExceptions
            }

            if (!string.IsNullOrEmpty(itemToDelete.RecurrenceRule) && itemToDelete.RecurrenceExceptions?.Count > 0)
            {
                // a recurring appointment was deleted that had exceptions, you may want to
                // delete or update any exceptions from the data source - look for
                // items where theItem.RecurrenceId == itemToDelete.Id
            }

            _data.Remove(itemToDelete);
        }
    }
}

        .no-allday .k-scheduler-head .k-scheduler-group:last-child{
            display:none;
        }

    <div class="no-allday">
        <TelerikScheduler Data="@Appointments" @bind-Date="@StartDate" @bind-View="@CurrView" Height="600px" Width="800px">
            <SchedulerViews>
                <SchedulerDayView StartTime="@DayStart" />
                <SchedulerWeekView StartTime="@DayStart" />
                <SchedulerMultiDayView StartTime="@DayStart" NumberOfDays="10" />
            </SchedulerViews>
        </TelerikScheduler>
    </div>

Error: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection