Today, we celebrate Holy Thursday. The Sacred Triduum begins at sundown with the celebration of the Mass of the Lord's Supper. It is the only Mass celebrated today, with the exception of the Chrism Mass.
In my diocese, as in many others, the Chrism Mass was celebrated earlier in the week, in order to make it more accessable for the clergy.
Today is the day we remember, in a special way, the events which took place at the Last Supper. We re-enact the washing of the feet, read the scriptures where Jesus instituted the Eucharist, and the clergy.
You will notice that today's Mass doesn't really end. The Eucharist is brought to the chapel of repose and the people just disperse. There is no recessional hymn or sign of the cross. Likewise, the Good Friday Liturgy begins and ends in silence and without the sign of the cross. In the same way, the Easter Vigil Mass also begins without a processional hymn and without the sign of the cross. However, it does end with the sign of the cross and a recessional hymn. If one pays attention to these details it becomes obvious that these three Liturgies are really one celebration which are spread out over three days.