GanttView enables users to highlight a specific day of the week by setting its back color. For instance, the weekends in the GanttView view can be highlighted in a different color to distinguish them from workweek.

You can subscribe to the PaintDay event of the C1GanttView class and set the BackColor property to customize the back color.
Below code snippet shows how you can set the BackColor of the day in the GanttView control.

private void C1GanttView1_PaintDay(object sender, C1.Win.C1GanttView.PaintDayEventArgs e)
{
     if (e.Date.DayOfWeek == DayOfWeek.Saturday || e.Date.DayOfWeek == DayOfWeek.Sunday)
       {
            e.BackColor = Color.MistyRose;
       }
}