Which Christmas classic is believed to have begun as a song for Thanksgiving?

Online Answer
Jingle Bells"Jingle Bells" is a classic song sung at Christmas time, but it didn't start out that way. First published in 1857, it was written by James Lord Pierpont, to be sung on Thanksgiving — not Christmas.
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Originally named 'The One Horse Open Sleigh', James Lord Pierpont composed the song in 1850 for his Thanksgiving Sunday school class. ... According to History.com, the jingling bells refer to the New England sleigh races that were popular in the 1800s.
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James Lord Pierpont1. J.P. Morgan's uncle wrote "Jingle Bells." Born in 1822, songwriter James Lord Pierpont composed the music and wrote the lyrics for the holiday standard.