The songs you have studied so far have all been written in major keys, for example, G major or F major. There is another kind of key called a minor key.
The song God Rest Ye Merry Gentlemen is written here in the key of E minor (Em), which has the same key signature as the key of G major. Songs in a minor key usually end with a minor chord.
On the recording there are three drumbeats to introduce this song.
You can now play the songs on pages 30 and 31 of Supplementary Songbook B.