Remember (Christmas) is written in the key of E Major.
According to the Theorytab database, it is the 5th most popular key among Major keys and the 5th most popular among all keys. Major keys, along with minor keys, are a common choice for popular songs. The three most important chords, built off the 1st, 4th and 5th scale degrees are all major chords (E Major, A Major, and B Major).
See the E Major Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!