Can you define a Long Weekend?

You can use DayPilotNavigator.BeforeCellRender event handler to add a custom CSS class to a day cell.

http://doc.daypilot.org/navigator/cell-customization/

Note that in version 7.7 the CssClass value will be prefixed with Theme/CssClassPrefix in the CssOnly. This is not consistent with the BeforeCellRender behavior in other controls and it will be fixed in the next release.

Example:

protected void DayPilotNavigator1_BeforeCellRender(object sender, DayPilot.Web.Ui.Events.Navigator.BeforeCellRenderEventArgs e)
{
  if (e.Start.DayOfWeek == DayOfWeek.Friday)
  {
      e.CssClass = "weekend";
  }
}