You can use the standard tooltip which is enabled by default (ShowToolTip = true) or enable an event bubble (an extended tooltip).

The default tooltip text is set to the event text and you can customize it in BeforeEventRender event handler.

JavaScript

Since 8.3.2806 the event bubble is enabled by default and the tooltip is supressed. In order to switch to the native browser tooltip you need to set .bubble property to null:

<div id="dp"></div>
<script type="text/javascript">
  var dp = new DayPilot.Scheduler("dp");
  dp.bubble = null;
  dp.events.list = [
    {
      start: "2013-03-25T00:00:00",
      end: "2013-03-25T12:00:00",
      id: "123",
      resource: "A",
      text: "Event",
      toolTip: "my tooltip"
    }
  ]
  // ...
  dp.init();
</script>

ASP.NET WebForms

protected void DayPilotScheduler1_BeforeEventRender(object sender, BeforeEventRenderEventArgs e)
{
  e.ToolTip = "my tooltip";
}

ASP.NET MVC

protected override void OnBeforeEventRender(BeforeEventRenderArgs e)
{
  e.ToolTip = "my tooltip";
}