I wrote about Julia Wilbur's first Thanksgiving in Alexandria in my book. Here's a little more background.

On Thanksgiving 1861, she lived a quiet life in upstate New York. A year later, she was in the midst of the war.

On November 27, 1862, Union-occupied Alexandria celebrated Thanksgiving. The Alexandria Gazette reported that evening,

A local man named Henry Whittington scoffed at the notion at the time, writing in his diary that "our folks seem little inclined to follow [the] Yankee lead." Virginia had celebrated Thanksgiving earlier in the century, but in the 1850s, Gov. Henry Wise attacked the holiday as a way to spread New England-type values.

Julia Wilbur had lived in Alexandria less than a month. A last-minute invitation from a minister named Mr. Warwick came to celebrate the day at the Paroled Prisoners' Camp. 

She went along with Warwick; Jennie and Nettie Kimball, two young women whose parents ran the rooming house where Julia was staying, and an acquaintance named Mrs. Winsor.

From her diary: "The drive was invigorating, in spite of all the dead horses we saw along the way."

Once there, Col. Gabriel de Korponay, the camp commander, warmly greeted them and his other guests, including Alexandria's Military Governor John Slough and his wife. A Thanksgiving service, presided over by Mr. Warwick, was required for all those healthy enough to stand up (about 2,000 in all, according to her diary). "Before service, cheers were given for the President, Gen. Slough & Mrs. Slough. After service an avenue was christened Mrs. Bell Slough & more cheers were given."

After that,

"a very fine lunch was prepared in the Mess room, & spiced with sparking champagne....The Turkey was not cooked, & few people like raw turkey. Some things were well enough—justice was done to the wine by officers & guests....Ever so many toasts were drank & a few speeches made."

As for the rest of the camp--

"The sick had Oysters & Pies to day, but I think the rest of the soldiers had nothing extra. Poor fellows. If what was spent for the wine that these officers drank had been laid out in something for all the soldiers to remind them of their homes! Oh! I do pity them, & felt that it was hardly right for me to enjoy myself all day when there is so much suffering around me...."

A few notes of context:

• Lincoln signed a proclamation marking Thanksgiving as a national holiday the following year, in 1863. 
• Early in the war, both sides had Camps of Paroled Prisoners. In this system, each side housed its own POWs that the other side had taken: in other words, this camp housed Union men captured by the South. The parolees would not take up arms until they were exchanged for prisoners taken from the other side--giving their "parole" or promise not to return to battle. So they languished in camp, a blessing or a curse depending on one's view of combat or guard duty. Later in the war, prison camps were established, including the infamous camps in Andersonville and Libby (for Union prisoners) and Elmira (for Confederate prisoners).
• Col. Gabriel de Korponay had a colorful life. He emigrated from Hungary in 1844 and was one of several people who claimed to have introduced the polka to the US. Earlier in 1862, he commanded a Pennsylvania regiment; he was popular with his men but considered "a drunkard and generally unfit to lead," according to First Sgt. Ambrose Henry Hayward (his letters edited by Timothy J. Orr and published by U. of Tennessee Press). He was transferred to the Parole Camp, presumably to shove him out of the way, although he certainly seemed to relish some of the perks of this position.