Really it´s spectacular. People from Portugalete takes free day to live the 'La Guía' (the guide) and most of them the next day too. It´s a very beast party, cause of alcohol! Really, very funny experience.
To arrive to Portugalete you can take Metro´s line 2 or Cercanías RENFE. You can also take Metro´s Line 1 to Areeta and cross the Estuary.
This happens at 12:00 h.