Freedom. Artistic freedom is the most important theme of this story. The nightingale has a good voice as it roams in the nature. Initially princess September does not keep the bird locked up in a cage, but, lets it to be free. BUT, due to the brainwashing of September's sister, she locks her the nightingale in a golden cage and does not allow it to be free. She doesn't realize the fact that every living creature has the right to live and be free. The bird is not free  becomes sad and does not sing for the princess. One day, September finds the bird lying motionless in the cage, take it out and the bird speaks to her and tells her that he cannot sing unless he's free. Pricess September now, learns her lesson and lets the nightingale free. From that day on, the bird came to her to sing a beautiful song everyday. This lesson teaches the readers about artistic freedom and the friendship between the nightingale and Princess September.
artistic freedom:- this is the most important them of the story'the princess September'.the author says that real artistic work can be done only in the  state of the perfect freedom.