In
DayPilot Pro 4.5 (see also) the Bubble was generalized and it now shows ToolTips for other objects as well (time cells, headers).
You should cast the
RenderEventArgs to
RenderEventBubbleEventArgs (see example from Demo/Calendar/Default.aspx.cs):
if (e is RenderEventBubbleEventArgs)
{
RenderEventBubbleEventArgs re = e as RenderEventBubbleEventArgs;
re.InnerHTML = "<b>Event details...</b>";
}