The song is was written by a Ukrainian and is called Shchedryk. It was originally a New Year's song; when Shchedryk was written, the Ukrainian New Year was in April, so it's actually a springtime song, and has nothing to do with bells.
There are some simply fantastic recordings of Shchedryk sung in Ukrainian; although (or maybe because?) I don't understand Ukrainian, I find these more beautiful and moving than the English lyrics.
Edit: several articles (words) were dropped, but only articles. So I have either suddenly a weird sort of brain disease that affects only some parts-of-speech, or ... well, that's the only reasonable explanation. Anyway, edited to fix.
Was lucky enough to go to one of the Eurovision shows in Liverpool this year and they had the Ukrainian version as part of the half time show. I could listen to it every day of my life so it's handy that there's a version for every season!
Do you know if it was based on any plain chant roots? The ostinato shares a basic note structure with the Dies Irae, (Day of Wrath) and I've been wondering if they were connected.
I saw trans Siberian orchestra live a few years back, and it was terrible. It was nothing like Carol of the bells (in terms of energy and being good) and I ended up leaving halfway through the show.
This here is in case you listen to the song, and it sounding like a boss battle just WASNT enough, well, heh, EVERY Lindsey Sterling video looks like a pre-boss battle cut scene lifted from Final Fantasy.
I always expect lasers or explosions to happen at the end, commencing the start of the battle.
....i think my joking about that ruined Lindsey Stirling for my wife.