Independence Day is a holiday celebrated in Bikini Bottom. It marks the anniversary of the day that Bikini Bottom gained independence from Bikini Top.
According to Sandy and Plankton, there are fireworks involved (Plankton referred to it as "explosives"). The fireworks may have come from the Fireworks Storage.
On, at least, the 2014 Independence Day, the Krusty Krab sold Krusty Dogs.