CardDAV uses VCard 3 by default, i.e. CardDAV clients and servers MUST understand VCard/3. VCard/3 defines BDAY (birthday) but not ANNIVERSARY.
DAVdroid doesn’t support VCard/4 (yet). VCard/4 defines ANNIVERSARY.
So, there are multiple ways to implement ANNIVERSARY with VCard/3:
- use ANNIVERSARY even if it’s not defined (the way used by DAVdroid), because unknown properties must be ignored by clients/servers – this is ,
- use X-ANNIVERSARY – this is used by posteo,
- use other field names – this is used by iCloud, they seem to name the fields “itemN.X-ABDATE”.
Conclusion: Currently, it’s not possible to sync ANNIVERSARY reliable with DAVdroid because with VCard/3, there are different methods to implement ANNIVERSARY and DAVdroid can only use one of them.