From The Art and Popular Culture Encyclopedia
A block party is a large public celebration in which many members of a single neighborhood congregate, either to observe an event of some importance or simply for mutual enjoyment. The name comes from the form of the party, which often involves closing an entire city block to vehicle traffic. Many times, there will be a celebration in the form of playing music and dance. Block parties gained popularity in the United States during the 1970s. Block parties were often held outdoors and power for the DJ's sound system was taken illegally from street lights. This was famously referenced in the song "South Bronx" by KRS-One with the line:
"Power from a street light made the place dark. But yo, they didn't care, they turned it out."
Traditionally, many inner city block parties were actually held illegally, because they did not file for an event permit from the local authorities "if needed". However, police turned a blind eye to them.
In the suburbs, block parties are commonly held on holidays such as Fourth of July or Labor Day. Sometimes the occasion may be a theme such a "Welcome to the Neighborhood" for a new family or a recent popular movie. Often block parties involve barbecuing, lawn games such as Simon Says and group dancing such as the Electric Slide, the Macarena or line dancing.
In other usage, a block party has come to mean any informal public celebration. For example, a block party can be conducted via television even though there is no real block in the observance. The same is true for the Internet.
A block party often includes a fire hydrant spouting water at the neighborhood children. In many small towns, the local fire department may also participate in the party, bringing out trucks that they display for show.