Some info about camping in the Black Rock Desert.
4th of Juplaya isn't an organized event, or even any kind of event. Some people like to visit the playa of the Black Rock Desert, near Gerlach, Nevada, at various times of the year, including 4th of July (hence "Juplaya") to celebrate Freedom, America, and the great American outdoors. Large, organized gathering require a permit from the BLM, but individuals and small groups can just drive out and pitch a tent for a few days without a permit.
If you've never visited the desert before, do some research before heading out there, to avoid causing harm to yourself, other people, or the desert environment itself. The desert can be a harsh environment. Do some web searches. Check the forums at the link below.
(stick a '2' on the end of the username to get the right email address)