I have fully working IPv6 connectivity (10/10 by test-ipv6.com metrics), but ipv6day.org still fails for me. It seems that the problem is that there're no A/AAAA records:

    $ host -t AAAA ipv6day.org 
    ipv6day.org has no AAAA record
    $ host -t A ipv6day.org  
    ipv6day.org has no A record
    $ host -t MX ipv6day.org 
    ipv6day.org mail is handled by 20 mail2.consulintel.es.
    ipv6day.org mail is handled by 30 dns1.consulintel.com.
    ipv6day.org mail is handled by 10 mail.consulintel.es.
    $ host -t SOA ipv6day.org 
    ipv6day.org has SOA record ns1.euro6ix.com. dnsadmin.consulintel.es. 2010120100 86400 7200 2592000 172800