In the Roman Catholic, Orthodox and in many Protestant churches, the Triduum is the deepest and most somber period of the Christian calendar. Not technically a part of Lent, it is three days of Holy Week: Holy Thursday (Maundy Thursday), Good Friday, and Holy Saturday. I will describe these from the Catholic perspective, since that's what I'm most familiar with.
Holy Thursday commemorates the Last Supper and the Agony in the Garden of Gethsemane (Where Jesus prayed all night and asked God to 'take this cup from Me'), and consequentially coincides with the Jewish Passover. The Passion of Christ is read through again during the Gospel, but I don't believe it's as long as the version read on Palm Sunday, the beginning of Holy Week. The Last supper is commeorated by the washing of the feet of twelve parishoners, to represent the washing of the feet of the Twelve Apostles, a very important gesture in Middle Eastern Culture at this time. The Holy Thursday celebration ends with a Benediction ceremony and a stripping of the altar and emptying of the tabernacle. The church empties in silence, and the most somber period has begun. Sometimes, a Passover Seder supper is celebrated immediately after the mass.
Good Friday is another very somber day, a day of fasting. The death of Christ is always celebrated at 3 pm, the time the Gospel tells us that He said 'into Your hands I commend my spirit' and breathed his last. The service is not a mass, for teh Eucharist is not celebrated. Communion is given out, but it has to have been consecrated the night before. A procession goes about the church, with the Cross and incense, with the words 'Behold behold, the wood of the Cross, on which is hung our Salvation' sung at intervals, followed by a kissing of the feet on the Cross. That night the Stations of the Cross are usually held.
Holy Saturday is the day of the most beautiful and longest of all the Catholic Masses. It begins with all of the lights in the church extinguished, and the Pascal Candle is lit, from which among lots of joyous singing individual candles are lit, illuminating the whole church. It is at this mass that the Litany of the Saints is sung, and it is at this mass that new converts to Catholicism are officially baptized and confirmed as Catholics. The Eucharist is once again celebrated, and the mass ends in joyous celebration: Easter is here, Christ has risen. So ends the Triduum and Easter season starts.