Christmas 2020 - Good King Wenceslas - Loreena McKennitt
The song is propoganda; Wenceslas I wasn't a king, and the song has nothing to do with reality. He was a good man and a Christian martyr though, and the song is top-shelf. Michaelmas was the day after Christmas (day 2 of the 12 days), so it qualifies as a Christmas song. The last verse of the song also confers some gravitas to the song.
I am fond of Loreena McKennitt's version of the song because it seems more traditional and "older" than a lot of modern versions.