You can add active areas to day cells.

JavaScript

javascript monthly calendar cell active areas

This example adds a green bar to the bottom of today using an active area:

const calendar = new DayPilot.Month("dp", {
  onBeforeCellRender: args => {
    if (args.cell.start === DayPilot.Date.today()) {
        args.cell.properties.areas = [
            {
                left: 0,
                right: 0,
                bottom: 0,
                height: 20,
                backColor: "#6aa84f",
                fontColor: "#ffffff",
                text: "Today",
                horizontalAlignment: "center"
            }
        ];
    }
  }
});
calendar.init();

ASP.NET WebForms

Not supported.

ASP.NET MVC

Not supported.