Thanksgiving Day was a well-established tradition in New England at the time of the Revolutionary War. The government of each colony would generally proclaim a “Publick Thanksgiving” each year on a Thursday in late November or early December. It was often on different days in different colonies; for example, in 1775, Connecticut held it on November 16th, Massachusetts and Rhode Island on the 23rd, and New Hampshire on the 30th.
Food seems to have been part of the tradition, but churches were also encouraged to hold worship services on that day, and people were encouraged to attend and (of course) to literally give thanks to God.
In Massachusetts in 1775, it was only natural that the Thanksgiving proclamation dwelt a lot on how the war was going. Among other things, it encouraged people to thank God “That the Lives of our Officers and Soldiers have been so remarkably preserved, while our Enemies have fell before them … And to Offer up humble and fervent prayer to Almighty God for the whole British Empire, especially for the United American Colonies”.
General George Washington ordered the American soldiers at the siege of Boston to observe Thanksgiving as proclaimed by the colony of Massachusetts. But there were still military duties to be done. Here’s how one American lieutenant, Jabez Fitch, described the day in his diary:
The 23d. This is Thanksgiving Day in this province. After breakfasting on chocolate and bread and cheese I went on the duty of fatigue. Our regt [regiment] were assign’d with Col. Wyllys’ to cut apple trees and make a brush fence from our front on the right of the lines down toward Dorchester, and we were stinted to extend it this day as far as the next intrenchment, which we accomplish’d by about 2 o’clock. We were directed in the work by one Lt. Cole of Wyllys’ regt, and after we had done work he came home with me calling in at the main guard, &c. After we came into camp we had a very good dinner on a piece of roast pork and a turkey, which we had prepar’d for that purpose. Capt. Bissell, Lt. Cole, Mr. Hillyer, Lt. Gove and I din’d together, and in the evening all of us, except Lt. Cole, went up to Jamaica Plain to make Capt. Rowley a visit, we also found Lt. Gillett there, he sung us several songs, made us a shoe, &c. A little after 8 o’clock we came home, had orders to turn out on the shortest notice, as an alarm was expected this night on account of our people beginning to intrench on Cobble Hill.
(I’m guessing that the phrase “to make a shoe” meant “to dance.”)
Interestingly, in addition to all the thanks and prayers that Massachusetts said should be offered regarding the war, their proclamation ended by encouraging people to pray
That he [God] would graciously pour out of his Spirit upon all order of men through the land, bring us to a hearty Repentance & Reformation Purity and Sanctify all his Churches
That he would make ours Emanuels Land
That he would spread the Knowledge of the Redeemer through the whole land and fill the World with his Glory, and all servile Labour is forbiden on said day.
GOD SAVE THE PEOPLE
Sources
Smith, Charles C., and Samuel A. Green. “May Meeting, 1894. Diary of Jabez Fitch, Jr.; New Volume of Proceedings.” Proceedings of the Massachusetts Historical Society 9 (1894): 40-95. www.jstor.org/stable/25079765. Page 83. https://www.jstor.org/stable/25079765?seq=44#metadata_info_tab_contents
The Acts and Resolves, Public and Private, of the Province of the Massachusetts Bay, vol. 19 (vol. 14 of the appendix), pp. 136-37.
https://archive.org/details/actsresolvespass7576mass/page/136
“General Orders, 18 November 1775,” Founders Online, National Archives, accessed September 29, 2019, https://founders.archives.gov/documents/Washington/03-02-02-0362. [Original source: The Papers of George Washington, Revolutionary War Series, vol. 2, 16 September 1775 – 31 December 1775, ed. Philander D. Chase. Charlottesville: University Press of Virginia, 1987, pp. 392–393.]