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The Hunger Games were established at the end of the Dark Days when District Thirteen held an uprising and rebellion against the Capitol. The Games are a punishment for the rebellion and a warning for future residents of Panem that something worse may come their way if they rebel again.
The Reaping happens once a year, a few months before the Games begin. A person's name will automatically be submitted into the Reaping once he or she is twelve years old, and will no longer be submitted under one of two conditions: 1) Said person has turned 18, and is no longer eligible for the games, or 2) they had been Reaped already and won the games. People will also have a chance to enter their names into the Reaping more than once in exchange for goods.