In the book, Katniss Everdeen sometimes mentions the Dark Days but reveals little. The government is careful in its presentation of what happened during the Dark Days. Very little is known about this time period.
This time period was known as the first rebellion, around the time of young Mags, who was the age of 5 when the Games started and around 80 when she volunteered for Annie Cresta at the 75th Hunger Games. The Hunger Games started after the first rebellion. During the Reaping for the 74th Hunger Games, Mayor Undersee gives a speech he gives every year, describing the history of Panem. He says that 12 of the districts were defeated and the last one, District 13, was destroyed.
As punishment for the rebellion, each district was to send one boy and one girl to the Capitol to take part in a brutal event called the Hunger Games. This was part of the peace treaty that all the districts signed, both to stop the rebellion and give the Capitol citizens some entertainment, as they didn't have to sign up themselves. This document was known as the Treaty of Treason.
During the second rebellion, President Snow sent numerous propaganda just like the rebels,but explaining the consequences of their actions, and how harsh they would be, should the rebellion fail.