I had 2 accounts configured on my Mac, iCloud (mail / calendar / contacts disabled) and Google (calendar / contacts enabled). Starting from some point, no idea when it started happening and what triggered it, all birthdays were duplicated in "Birthday" Calendar. Today I've got annoyed by this and completely deleted Google account from my Mac, which removed one set of birthdays from Calendar, but only one, second set of birthdays is still visible in "Birthdays" calendar on my Mac.
Now there is only iCloud account left, Calendar / Contacts are disabled there. In preferences of both of Calendar / Contacts I can see my iCloud account, but it's disabled in both places. Contacts.app is empty, there is just "All contacts" section on the left site without any contacts in it. No appointments in Calendar.app as well, but I still see birthdays of all my Google accounts. Tried to disable "Show birthdays calendar" and enable it again - birthdays are still there. Tried to delete everything in ~/Library, ~/Library/Cache and ~/Library/Preferences which has something with Calendar or iCal in its' name, started Calendar.app - birthdays are still there.
I've even checked iCloud Web UI, there are no birthdays, no appointments, no contacts, so cloud part is completely empty, means birthdays are cached / stored somewhere on my Mac, so, the question is where they are cached / stored and how to remove them?