Why is the last word passée instead of passé?

It is passée because the direct object precedes the past participle.  "journée" is the direct object and is feminine.

It would be passée if the subject is feminine. Passé on the other hand is for a masculine subject.

-Adding es is feminine and plural

-Adding e is feminine 

-Adding an s is masculine and plural

L'objet de passer, belle journée, est avant le verb.