There's a calendar built-in to vBulletin. Members can add events to it. No one really does it, but I guess that's there for when someone does. I think it's public whenever anyone makes an event, so you can't use it as a personal calendar or anything.
Anyway, when there's an event (that isn't a holiday automatically added to the calendar by vB), you can request to subscribe to it. Then it will send you an automatic reminder for it. Those reminders would be listed in that section of the User CP. Since the calendar isn't really used on PC, you probably won't be using the feature much.
