@herz_ag I see. I’m afraid this can’t be done either with standard CardDAV/vCard (contacts must have exactly 0 or 1 BDAY), nor with most Android contact apps, which don’t allow do set more than one birthday per contact. Other event types than anniversary and birthday would be supported by Android, but are not supported by vCard and could only be implemented over DAVdroid-specific X- properties, which would not be supported by other clients. Maybe this will be implemented in the future, but at the moment, there is no priority for that.
So for the moment, I recommend to create multiple contacts with one birthday per contact.