Thanksgiving used to be on the last Thursday, but they moved it to the fourth Thursay to give more shopping time before Christmas, to help the economy. I think this happend during the Roosevelt (FDR) administration.
Voting was originaly on the first Monday in November, but paydays were often the first of the month, and people wanted to blow their money quickly, and ended up not voting when Monday was the 1st. So they moved it to the Tuesday following the first Monday to avoid this.
These are the stories I heard, and they sound plausible. I’ve never heard any competing ideas.
Oh, why Thursday for Thanksgiving? I don’t know.
It is too clear, and so it is hard to see.