I'm Looking Forward To Joining You Finally is written in the key of D Locrian.
According to the Theorytab database, it is the 2nd most popular key among Locrian keys and the 73rd most popular among all keys. The D Locrian scale is similar to the D Minor scale except that both the 2nd and the 5th notes (E♭ and A♭) are a half step lower. This means that the characteristic "home base" chord of D Locrian has a dissonant diminished quality (d˚), which makes it a very uncommon choice for songs.
See the D Locrian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!