Gantt view: Add a definable holiday (or other conflict) task bar, with option to have semi-transparent colored column go all the way down the entire gantt view, to immediately show holiday impact across the entire schedule.

Example: User defines a company retreat period spanning e.g. 4 days, makes it red, 50% transparent, and it appears as vertical red bar 4 days wide, from top to bottom. Makes it visually obvious where tasks are crossing a possible conflict period.