Finn has the Grass Sword in season 5, episode 20 if you mean the sword he’s using around that point in Adventure Time. The show’s sword history places the Grass Sword as Finn’s main weapon starting after the Demon Blood Sword is broken, and before he later gets the Finn Sword.

Sword context

  • Before this point: Finn used the Demon Blood Sword until it was broken.
  • After that: he switches to the Grass Sword, which becomes his main sword in season 5.
  • Later upgrade: he eventually gets the Finn Sword in season 6.

Note

Episode numbering can vary a bit by source and region, so if you meant a different “episode 20” within a later season, the sword might be different.