Subtle change to Calendar app in iOS 6.1?
I'm not 100% sure this is new to iOS 6.1, but I've never had this happen to me before.
When you tap and hold a date on the calendar in month view, the Add Event page comes up. I discovered this by sliding my thumb around on the calendar, as I often do when I'm bored. (I get bored a lot, so I think I would have found this feature had it been in previous versions of iOS.)