Thanking God is considered to be a private act, so what business does government have creating an official holiday--Thanksgiving--specifically to thank God? Does the First Amendment prohibit the government establishment of religion and guarantee the separation of church and state?
Considering the secularism and the growing hostility that's increasing gripping the country, the question is inevitable, if not already being asked.
The Thanksgiving holiday clearly has its origins in faith in God and religious celebration. The first Thanksgiving was held in the autumn of 1621 by the Pilgrims--a distinctly religious sect. For three days, they were joined by members of the Wampanoag tribe in a feast of fowl, deer and fish.
In 1777, the nation that was birthing during the War of Independence celebrated a day of Thanksgiving and in 1789 President George Washington proclaimed Thursday, Nov. 26 to be "a day of public thanksgiving and prayer."
Next came Sarah Joseph Hale who used her publication, "Godey's Lady?s Book" to petition presidents Taylor, Fillmore, Pierce, Buchanan and Lincoln to designate Thanksgiving as a national holiday. In 1863, Lincoln agreed, designating the last Thursday of November "as a day of Thanksgiving and Praise to our beneficent Father who dwelleth in the Heavens."
In 1939, President Roosevelt agreed with retailers to move the holiday up a week to increase the shopping days before Christmas. He proclaimed:
It is a good thing to give thanks unto the Lord." Across the uncertain ways of space and time our hearts echo those words, for the days are with us again when, at the gathering of the harvest, we solemnly express our dependence upon Almighty God.
Washington, Lincoln and Roosevelt could never get away with any of that today. I haven't heard of anyone demanding that government withdraw its designation of what's basically a religious holiday, but I wouldn't be surprised that I live to see it.
We see assaults on supposed government endorsement of religion all around us: From demands that children stop saying the Pledge of Allegiance in school because it contains the loathsome words, "under God" to demands that military chaplains be fired because they espouse their belief that homosexuality is a sin. Of course, it's hard to ignore the Obama's administration requirement under its interpretation of Obamacare that people of faith pay for things that violate their consciences. (Here are questions of Obama's hostility to religion.)
One must ask, what harm does it do to have a government-designated day of Thanksgiving? As opposed to what good does it do? Being a person of faith, I think it does a lot of good, offering as a nation our thanks to God for the blessings that he has bestowed on us.
Related: Here's an interesting video about what an atheist should do when he is asked to offer a prayer of thanksgiving at a family dinner: