The Purge is an annual event within The Purge Film Series. It is in fact the main setting for where much of the film's action takes place.
After an economic collapse in the United States back in 2014, The N.F.F.A overthrow the old government. This results in the establishment of a police state and totalitarian regime, but by 2017, they ratify the constitution's 28th Amendment, establishing the Purge.
The Purge is within the month of March, however doesn't start until March 21st at 7:00 pm that night. Before such an event occurs, the Emergency Broadcasting System will appear, making everyone aware of the beginning of the purge. The next is the sounds of sirens being sound throughout the areas where the purge is taking place and as a result, the purge begins. This gives anyone looking to perform criminal activities to do so for the next 12 hours before 7:00 am, which by that time, all crime will be dubbed illegal once more.
During the purge however, any form of emergency services (Police, Fire, Medical) are unavailable until the next morning and the use of class 4 weapons are authorized, such as the use of small explosives, automatic weapons, machetes, etc. Weapons however above that class are deemed illegal and violation of such will result in public execution by hanging. As far as government officials go however, those with a ranking 10 or that of a higher ranking are dubbed immune to the effects of the purge, however in The Purge: Election Year, this rule was revoked.