The Holy Fire is a miracle that ocurred for centuries in the tomb of our Lord Jesus Christ, when True Orthodox Christians celebrated the service of the resurrection.
Every year, at the start of the Sunday of Reurrection, at midnight (sometimes earlier, sometimes latter) Holy uncreated Fire emerged from the Tomb of our Lord and God, Jesus Christ, and went to different places in the Church, many times, enkindling the candles of believers directly.
The service of the Holy Fire in the Tomb of our Lord Jesus Christ is now officiated by heterodox clergy, and the miracle does not happen in their service. The Holy Fire stopped appearing somewhere in the middle of the past century (50's or 60's) after heterodox took over the place and banned the True Orthodox Christians from the service.
In remembrance of that miracle, in Churches round the world, the service of the Holy Fire is officiated to mark the starting on Pascha (Holy Resurrection Feast) and it's done exactly at midnight.
Before midnight, there are some special prayers, and then the lights of the Church are turned out, the officiating clergy light a candle in the altar, and come out with the Paschal greeting, from the main door (Royal Gates) and approach the others to enlighten the candles.
The moment the officiating clergy comes out with the candle, all lights in Church are turned on, the choir and all present begin to sing, and the bells are rang in what is called "joyful peal". And all the doors of the altar are opened.
People greet each other with the paschal greeting, we hug and kiss, people we know and strangers for the same, for we know we are all brothers and sisters in Christ.