A program note is a way to provide information to the audience to enhance their experience.
Good program notes also provide a service to directors, saving them the time and energy before a concert to come up with their own.
Tips:
Some directors might use students to announce the music at a concert. Consider what that will sound like using your program notes and write the notes in third person.
Program notes are not a sales pitch. The music has already been purchased. Keep the information relevant to the general audience.