Somebody please prevent this error

1. If you want to use this event handler to customize the Navigator cells you need to add the method to the code behind:

    protected void DayPilotNavigator1_OnBeforeCellRender(object sender, BeforeCellRenderEventArgs e)
    {
    }

2. If you don't want to handle the BeforeCellRender event you need to remove the following attribute from <DayPilot:DayPilotNavigator> tag:

OnBeforeCellRender="DayPilotNavigator1_OnBeforeCellRender"

I assume you are working with the Doctor Appointment Scheduling tutorial (https://code.daypilot.org/55027/asp.net-doctor-appointment-scheduling). You can download a working Visual Studio project there (see the link in the "Downloads" section at the top of the tutorial).