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";
        }

    }