Prior to Lincoln's Thanksgiving Proclamation, the holiday was celebrated by many states throughout the Union. However, the states which chose to participate in celebrated on different days, as scheduled by that state.

On September 28, 1863, Sarah Hale, the 74 year old editor for Godey's Lady Book, wrote a letter to President Lincoln requesting that one day be singled out so that all the states within the Union could remember Thanksgiving on the same day.  

She requested him to have the "day of our annual Thanksgiving made a National and fixed Union Festival." She explained, "You may have observed that, for some years past, there has been an increasing interest felt in our land to have the Thanksgiving held on the same day, in all the States; it now needs National recognition and authoritive fixation, only, to become permanently, an American custom and institution."George Washington was the first president to proclaim a day of thanksgiving, issuing his request on October 3, 1789, exactly 74 years before Lincoln's.

In response, Lincoln issued this proclamation:

After the Battle of Shiloh on April 6th and 7th, 1862 in Tennessee, over 16,000 wounded soldiers lay in the rain and cold mud for over two days as overwhelmed doctors and nurses struggled to locate and treat the soldiers.
Some of these wounded soldiers later reported that as they lay on the ground awaiting help, their wounds started to glow in the dark.
At the time, the reason for the glow was a mystery but doctor’s did note that the wounds that glowed healed faster than those that didn’t.
The mystery remained unsolved until 2001, when two teenagers finally uncovered the source of the glow.
After the two teens, Billy Martin and John Curtis from Maryland, conducted a variety of scientific experiments, they discovered that the wounded soldiers became hypothermic as they lay in the mud.
This lower body temperature allowed for the growth of a bioluminescent bacterium called Photorhabadus luminescens, which inhibits pathogens, to develop in the wound.
This bacterium not only caused the wounds to glow but also prevented them from became gangrenous, which saved the lives and limbs of many soldiers.

As war descended on Virginia, Lee and his wife Mary fled their 1,100-acre Virginia estate, known as Arlington, which overlooked Washington, D.C. In 1863 the U.S. government confiscated it for nonpayment of $92.07 in taxes. Meanwhile, Lincoln gave permission for a cemetery to be built on the property, including a burial vault on the estate’s former rose garden.

The idea was that, should Lee ever return, he would “have to look at these graves and see the carnage that he had created,” according to his biographer Elizabeth Brown Pryor. After the war, the Lees quietly looked into reclaiming Arlington but took no action before they died.

In 1877 their oldest son, George Washington Custis Lee, sued the federal government for confiscating Arlington illegally; the Supreme Court agreed and gave it back to him. But what could the Lee family do with an estate littered with corpses?

George Lee sold it back to the government for $150,000. Over time, 250,000 soldiers would be buried in what is now Arlington National Cemetery.

 (Copied from http://www.history.com/news/10-surprising-civil-war-facts)

For more information, see also http://www.smithsonianmag.com/history/how-arlington-national-cemetery-came-to-be-145147007/?no-ist=&page=1

"As a member of the military force of the State, I promptly responded to the call to arms, to maintain the public peace, and to prevent passage of northern troops through its territory, and when the efforts of our authorities became futile, and their functions were usurped by the federal government; when it became apparent, without question, that the hope of State action was impracticable, by reason of this military occupation, then, without hesitation, I chose to cast my fortune with the south and to bear a part in the great struggle."

In current times, the Civil War is understood to be Anti-Slavery versus Pro-Slavery, and while the institution of slavery was certainly and undeniably a key factor in the development of the war, the fullness of the discord between North and South were far more complex. Slavery as the defining character of the war did not emerge until 1862 and the issuance of the Emancipation Proclamation, which freed only the slaves within the states of the supposed rebellion. At the outbreak of the war, the South was fighting for the right of the States to create and maintain their own laws, with the intention of maintaining the original intent of the constitution that the federal government existed not to dictate to the individual states but to oversee and protect them. The North's position was that it was unconstitutional for the states to secede from the Union, stating that the language of the constitution implied that the states which had joined the Union had done so to perpetuity.

George Booth explains it this way: "My early reading and associations led me to take views of the great questions which agitated and disturbed the public mind, in the days of 1860, by enlisting most strongly my sympathies in favor of rights of the States under the constitution, and in opposition to efforts and the dominant purpose of the north to violate the express terms of that compact, and to destroy the principles of home government. With all this, in common with most Marylanders, was held in sacred reverence the love of the Union and the glories of our common country. The dissolution of the Union was looked upon as a threatened evil, to be averted by mutual concession and forbearance, and the efforts of those patriotic statesmen who so earnestly strove to prevent this dire calamity found in my heart a most responsive sentiment. After the lapse of many years, during which the crude thoughts and convictions of these earlier days have matured in character and strength, and in judgment which comes after experience, having been sanctified by trial and suffering, it is in no sense of vain-glory or boasting that I solemnly record that my mind and heart the more strongly justify the views of my youth, and my only regret is, that my slender abilities did not permit me to be more efficient in the defense of those principles which are the very foundation and bulwark of a State, whose chief glory and power comes from a government with the consent of its people."

Now, so far removed from these events, and having only the echoes of the past to enlighten us as to the fullness of both positions, I view myself as a detective attempting to uncover and understand both sides of the war. For the record, I would like to be clear in my personal view that slavery was a despicable institution and that God created all human beings, regardless of gender or skin color, in His image. As such, the sanctity of all human life should never be questioned.

I believe we can all stand in agreement that it was a blessing for slavery to be abolished within the United States, but a tragedy that it had to come as such a high cost. The question of how much power is constitutionally given to the federal government remains a relevant question to this day, and the effects of this war ripple down to us from history. If compromises could have been reached in 1860, consider what a different nation we would have today.

My hope and my prayer for our country is that we can learn to come together for the greater good and put aside the differences which could so easily divide us, but do so at the detriment of all involved. As citizens of the United States, each of us, regardless of our political affiliations, will have to face the future we create with the decisions we make today and justify the consequences to our children.