The event ID is accessible in DayPilotBubble.RenderContent after casting the EventArgs toRenderEventBubbleEventArgs (re.Value):
protected void DayPilotBubble1_RenderContent(object sender, RenderEventArgs e)
{
if (e is RenderEventBubbleEventArgs)
{
RenderEventBubbleEventArgs re = e as RenderEventBubbleEventArgs;
re.InnerHTML = loadDetailsForId(re.Value);
}
}
See also Demo/Calendar/Default.aspx.cs for an example.