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.

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";
}

JavaScript

<div id="dp"></div>
<script type="text/javascript">
  var dp = new DayPilot.Scheduler("dp");
  dp.durationBarVisible = true;
  dp.durationBarMode = "PercentComplete";
  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>