The home team in a final must be the older
Despite Barça will play Copa del Rey final against Athletic Club at Camp Nou, they won't 'feel' like home. Next May 30 Luis Enrique's team will face Athletic and Barça will be considered the away team.
According to the rules, the home club must be the older. FC Barcelona was founded on November 29, 1899, a year later after Athletic Club (1898). So Athletic might use Barça's usual dressing room unless both teams reach an agreement.
If there's no agreement, it will be the first time Barça play as guest on their own field. In another final played at Camp Nou, in 1963, they faced Zaragoza, a much younger team.