Just like Post-Apocalyptic Christmas part 1, part 2 relies on the use of familiar Christmas tunes. I did this to lend the song the sense that I was remembering fragments lost long ago.
In Post Apocalyptic Christmas Part 2 the piano introduction is a fragment of We Three Kings. This motif was the catalyst for the structure of the song - you can hear it come back after each verse but it gets 'reframed' in different ways using minor and major keys based on the narrative.
The next tune, O Come All Ye Faithful, forms the chorus. It is however in a minor key so the melody has been transformed a bit. The words have also changed and contain a play on words - instead of "come ye to Bethlehem" I sing "come ye despite mayhem" (I'm most proud of that one). The chorus melds into another Christmas standard, "O Holy Night". This isn't the melody of that song but the words, which I've inverted here: instead of "fall on your knees / O hear the angels voices" I sing "rise from your knees / O hear the winds are blowing".
The third verse contains the most buried Christmas fragment: after I sing "reviving olden ways" I give a nod to the first PAC that used a minor version of Jingle Bells. It's brief but it's in there!
In the big jumbled canon section near the end of the song I bring back text and melody fragments from We Three Kings (star of wonder / star of night). This all finally leads back to Come All Ye Faithful. At the end I rip into O Holy Night but this time it's the melody as it is in the original. There's some harmonic sleight of hand going on here but I'll save that for a different discussion.
Hope that helps you to identify all the old Christmas songs - listen again and see if you can pick all of them out now!