While Haakon is in your graveyard, you may cast it. This follows the normal rules and timing for casting a creature spell; the only difference is what zone Haakon is being cast from. The spell goes on the stack. You have to pay Haakon's mana cost and any applicable additional costs. The same applies for Knight cards you cast from your graveyard while Haakon is on the battlefield.

Haakon can't be cast from your hand. It can't be cast from any zone other than your graveyard, even if an effect such as that of Spelljack or Muse Vessel would otherwise allow you to.