Looks like HTC gets involved and offers continuous support for the HTC HD2 through their Support website, today releasing a second update this month after the re-release of the New SMS Function Update.
Today's update is called Update for HTC HD2 Annually Recurring Calendar Appointments and it is only applicable to International HTC HD2 devices (sorry T-Mobile HTC HD2). According to the description, "this update for HTC HD2 allows you to set recurring calendar appointments year on year". Well, to be completely honest, I don't quite use the Calendar on my HD2 to set up Appointments (I do that by setting them up on either of my PCs through Exchange so they get synched) but before applying this update, I was able to create an appointment on my HD2 that would repeat every July 28. Tell us your thoughts in a comment below!
As for installation, the procedure's the same: download the exe to your Device and run it from there. It will Soft Reset when finished. You'll have to reapply after a Hard Reset though.