For more than 1600 years at the Cermony of the Resurrection we have witnessing this event. Some they might call it miracle or mystery of the Orthodox Christians. This is the link regarding the histoy for the Holy Fire - www.holyfire.org/eng/history.htm
The Patriach enters the Holy Grave and in limited short time he comes out with the candles that light in a mysterious way. Then at the Temple of Resurrection we have the appearance of the Holy Fire that lights the candles on the Church and the candles of the faithful ones. You can touch this light without get burned for 33 minutes...
Many have question this mystery and claimed that is fake. But there are many aspects that show the opposite. First the Holy Grave is sealed by the Israel Police, and during the Holy Week no one can enters. The Patriach of Jerusalim before entering the Holy Grave he is being checked by the Israel Police, in case that he is carrying any suspicious item to light the candles
Plus there are representatives from other Christian Dominations to observe the miracle or mystery. There are Roman Catholics, Armenians, Maronites, just to mention few. The Holy Fire appears only to the Christian Orthodox Church, despite the fact that the other dominations are using the Temple. Once the Armenians tried to perform the Cermony of the Resurrection by not allowing the Patriarch to enter the Temple. As a result the Holy Fire came out from a column inside the Temple leaving a crack as a witness for those who question the miracle.
For us the Apostolic Catholic Orthodox Church Christians the Holy Fire is a witness for more than 1600 years that indeed the Early Apostolic Church stands there for 2000 years defending Christianity and keeping the faith as it was delivered by the Apostles and their succesors. Because for us Christ is a living God, that heals our souls and freeing us from the sin.
This Holy Week of Jesus Passion we are living Jesus, because Jesus is not just having knowledge of Him, it includes more than that. It includes faith and accepting the Blood and Flesh of our Saviour to become one with Him, and to invite Him to our live through the Holy Spirit. This is the faith that was kept through the last 2000 years that Jesus is the Light of this world.