It's already available.
See this demo page, Event 12:
http://www.daypilot.org/demo/Month/
A snippet from Demo/Month/Default.aspx.cs:
protected void DayPilotMonth1_BeforeEventRender(object sender, DayPilot.Web.Ui.Events.Month.BeforeEventRenderEventArgs e)
{
if (e.Value == "12")
{
e.ContextMenuClientName = "menu2";
e.StaticBubbleHTML = "test";
}
}