These other two answers are great, but I would just like to add some more interesting information. Most of this information comes from a great source titled "Murderous Games" on the website www.historytoday.com.
It took many years for historical reenactments to become commonplace.
The Colosseum was dedicated in AD 80 with 100 straight days of "games". In one single day over 3,000 men fought; along with another 9,000 animals being killed.
The Emperor Trajan, to celebrate his conquest of Dacia (roughly modern Roumania), gave games in AD 108-9 lasting 123 days in which 9,138 gladiators fought and eleven thousand animals were slain. The Emperor Claudius in AD 52 presided in full military regalia over a battle on a lake near Rome between two naval squadrons, manned for the occasion by 19,000 forced combatants. The palace guard, stationed behind stout barricades, which also prevented the combatants from escaping, bombarded the ships with missiles from catapaults. After a faltering start, because the men refused to fight, the battle according to Tacitus 'was fought with the spirit of free men, although between criminals. After much bloodshed, those who survived were spared extermination'.
Mass executions were very popular as can be seen from this excerpt from Seneca, a Roman senator and philosopher.
[He] tells of a visit he once paid to the arena. He arrived in the middle of the day, during the mass execution of criminals, staged as an entertainment in the interval between the wild-beast show in the morning and the gladiatorial show of the afternoon:
All the previous fighting had been merciful by comparison. Now finesse is set aside, and we have pure unadulterated murder. The combatants have no protective covering; their entire bodies are exposed to the blows. No blow falls in vain. This is what lots of people prefer to the regular contests, and even to those which are put on by popular request. And it is obvious why. There is no helmet, no shield to repel the blade. Why have armour? Why bother with skill? All that just delays death.
In the morning, men are thrown to lions and bears. At mid-day they are thrown to the spectators themselves. No sooner has a man killed, than they shout for him to kill another, or to be killed. The final victor is kept for some other slaughter. In the end, every fighter dies. And all this goes on while the arena is half empty.
Here is another interesting fact about The Colosseum. It could seat some 50,000 people. And it had the worlds first retractable roof. Somewhat like modern day stadiums, the Colosseum used a very intricate system of awnings which they called a "Velarium". This system of curtains protected the crowd from the elements such as the sun and rain. But the main use of the velarium was actually to create a ventilation updraft, and circulate the arena with a cool breeze.