Animate Dead is put into your graveyard as a state-based action.
Activate Phyrexian Altar by sacrificing Cavalier of Dawn, adding ({B} magic symbol) .
Cavalier of Dawn's last ability triggers, returning Animate Dead from your graveyard to your hand.
Repeat.
Notes
Optionally, you may use Cavalier of Dawn's ETB ability infinite times, destroying any number of nonland permanents and giving their controllers a 3/3 Golem for each one destroyed.