Although the player himself describes himself as a "cool-headed person", Aitor Larrazabal admits that Sunday will be a difficult day.
Sunday's match against "Atletico" will be the 390th and final appearance for this 33-year-old Spaniard, who has spent his entire 22-year career as a footballer wearing the club's jersey.
Larrazabal's career, which began when he was 11 years old - when he started attending the "Athletic" youth academy and rose through the ranks of the club until becoming a member of the main "Athletic" team.
Bilbao team coach Ernesto Valverde stated that Larrazabal, who has been Asier Del Horno's "substitute" for most of the season, will start from the beginning of the match on Sunday. However, saying goodbye will not be easier for the Spaniard.
"Sunday will probably be the hardest day of my life," said the player. "Now everyone is thanking me and showing respect for what I have done for Athletic, but in reality, I am very grateful to the club that made me who I am and for everything I have."
"The time I spent here is described by only one word - unforgettable," said Larrazabal sadly.
Meanwhile, "Athletic" fans have not lost hope that the player will not end his career and will help "Athletic" compete in the UEFA Europa League next year.