A curtain is coloured strip shown in the background of a gantt chart for a specified duration or specified dates.
Most common use would be to indicate a period of non working time such as christmas holidays.
I need to do the same thing, although the duration is still working time - I want to shade the background of the programme for a specific two week period so that the reader is aware of the school holidays (which impact what road construction activities can happen at that time on this project).