For young Ed McCree, enslaved on a thousand-acre Georgia cotton plantation, Christmas and New Year’s Day 150 years ago were like none he’d ever known. After time off for Christmas and feasts with young pigs, cattle, and peaches (still summer-sweet because they were packed in wheat straw and cottonseed that kept them fresh), New Year’s Day typically meant that McCree would be again forced to carry buckets of water to the men and women working in the fields.

The difference that December 1864? William Tecumseh Sherman’s Union Army had just marched across Georgia and reached the seaport of Savannah, offering it to President Abraham Lincoln as a Christmas present.

New Year’s Day 1865—and those in the two years previous—were among the most poignant and pregnant with new beginnings in American history. Ever since Lincoln had signaled his intent in September 1862 to declare slaves in rebel states emancipated as of New Year’s 1863, the possibility of freedom for African-Americans in the South had been hanging in the air, depending on the war’s progression.

African-American communities already held traditional church services on New Year’s Eve, but they took on a special meaning as the country welcomed in the watershed year of 1863, becoming the predecessors of today’s Watch Night services. In Baltimore, Washington, D.C., and the many free black communities in other cities and towns, African-Americans gathered to anticipate the moment the United States finally would declare itself at war with slavery and not simply disunion...