Each line is an anagram of the Christmas message spelled out diagonally in capital letters. Furthermore, the capitalized letters in each line remain in the same positions if the line is re-anagrammed back to the original Christmas message.
Card, L. E.
"A Bouquet of Christmas Anagrams,"
Word Ways: Vol. 4
, Article 19.
Available at: https://digitalcommons.butler.edu/wordways/vol4/iss4/19