It seems as though it is a bug that rarely occurs in iOS when all of a sudden, there is more than one Birthday calendar in iOS. When I noticed that this bug was present in my iPhone, I spent hours trying to find a solution, only to find nothing short of restoring. I did however, find many posts on macrumors.com and apple.com where different people were reporting this issue.
Many of these people blamed it on iCloud, and it does seem to have something to do with it. I only use iCloud for Documents and Find My iPhone. When I noticed this problem, I tried everything that was suggested on the above sites with no progress. I even tried deleting all my mail and calendar accounts, and I was still stuck with four, that's right four, birthday calendars.
I proceeded to search for where in the iPhone file system the calendar data was stored and then proceeded to delete these files. I then force-closed the calendar, then re-opened the calendar and voila, everything was back to normal with only one birthdays calendar.
Here are the steps:
1. Jailbreak your i-device. If you don't know how, refer to idownloadblog.com.
2. Install iFile in Cydia
3. Make sure that Calendar app is not running in the app switcher (force-close it)
4. Open iFile and delete the contents of /var/mobile/Library/Calendar
5. Open the Calendar app
That's it!
I feel that it's worth mentioning that after this process, one or two of my most recent contacts was all of a sudden missing. Maybe it is related to the bug.
Leave me some comments below if you have any questions.