I’ve heard somewhere that it wasn’t until sometime in WWII that Vicksburg held a 4th of July celebration.
Probably from Ken Burns' documentary series. There is some question about whether that's true. Does it refer to official city celebrations or to what people in the city were doing on their own? Did it mean nobody had the day off to eat hot dogs, or just that the city didn't have a parade or set off fireworks?
Celebrations of the 4th of July, the day of surrender, were irregular until 1947. The Vicksburg Evening Post of July 4, 1883, called July 4 "the day we don't celebrate", and another Vicksburg newspaper, the Daily Commercial Appeal, in 1888 hoped that a political victory would bring an enthusiastic celebration the following year. In 1902, the 4th of July saw only "a parade of colored draymen". In 1947, the Jackson Clarion-Ledger stated that the city of Vicksburg did not celebrate the 4th of July again until 1945, and then it was celebrated as Confederate Carnival Day. A recent scholar disagrees, stating that large Fourth of July celebrations were being held by 1907, and informal celebrations before that. A large parade was held in 1890. -- Wikipedia
What is known is that there were large public celebrations in 1945 and 1946 (which some preferred to call "The Carnival of the Confederacy") and again in 1947 when Eisenhower came to town. The city fathers, the newspapers and possibly the National Park Service wanted to play up the post war celebrations to attract tourists and raise the city's media profile.