The origins of American Thanksgiving are a bit more practical. At first, it was in October because that’s when the harvest was. Then it was changed to “always” be on November 25.
But it’s “moveability” got the Puritans and Protestants all a-flurry.
The Protestants didn’t want Thanksgiving to interfere with people being able to attend Sunday services. They also didn’t want it to be an excuse to (heaven forbid) let people off their Godly duties to cleanliness earlier in the week. (IE Monday washing) Thursday was generally a day where lectures were given in righteousness so it was a matter of swapping out a lecture with a meal, rather than having it “interrupt” the “natural order” of living.