What Christmas song is played during the organ solo of a famously long psychedelic rock song from the 1960s?

In-A-Gadda-Da-Vida by Iron Butterfly

But epic length it is, with brief verses at the beginning and end, and long instrumental solos in the middle. The drum solo is one of the first ever to appear on a rock recording; the organ solo sounds a little like the Christmas carol God Rest Ye Merry Gentlemen in spots. If you're in the proper frame of mind when it comes on, perhaps you can get into the groove with the band and stick with them for the whole thing. But it's also possible to experience In-a-Gadda-da-Vida as the worst sort of hippie excess, an endless slog that leaves you longing for the single version, which runs 2:52.

https://www.songfacts.com/facts/iron-butterfly/in-a-gadda-da-vida