Totally idle curiosity here. The attached image is a company "shared calendar" in Outlook. Its purpose is to track who's on vacation at any given time. It's called the "Gone Calendar". Everyone who takes a vacation adds it to this shared calendar.

I've removed the names for privacy reasons.

There is a purple shading on some of the days. One of the days is even shaded a darker purple than the rest of them. Why? I have no idea what this purple shading indicates. It's not related to any of my other calendars including my own personal calendar or any of the internet calendars I'm subscribed to. It doesn't appear to correspond with the specific vacation days of anyone on the Gone calendar, including myself. It doesn't appear to correspond with any known public holidays. It doesn't correspond with any internal company project schedules (that I know about anyway).

Any ideas?