It's possible to set the bubble HTML in advance so it isn't loaded from the server using BeforeEventRender event handler (use e.StaticBubbleHTML property).
protected void DayPilotMonth1_BeforeEventRender(object sender, DayPilot.Web.Ui.Events.Month.BeforeEventRenderEventArgs e)
{
e.StaticBubbleHTML = "Event details";
}
In DayPilot Month (http://www.daypilot.org/month.html), this features is available since build 6.5.2999.
See also: