The work is presumed to have been started around 1495–96 and was commissioned as part of a plan of renovations to the church and its convent buildings by Leonardo's patron Ludovico Storza, Duke of Milan. The painting represents the scene of the Last Supper of Jesus with his apostles, as it is told in the Gospel of John, 13:21. Leonardo has depicted the consternation that occurred among the Twelve Apostles when Jesus announced that one of them would betray him.

Due to the methods used, a variety of environmental factors, and intentional damage, very little of the original painting remains today despite numerous restoration attempts, the last being completed in 1999.

The Last Supper measures 460 cm × 880 cm (180 in × 350 in) and covers an end wall of the dining hall at the monastery of Santa Maria delle Grazie in Milan, Italy. The theme was a traditional one for refectories, although the room was not a refectory at the time that Leonardo painted it. The main church building had only recently been completed (in 1498), but was remodeled by Bramante, hired by Ludovico Sforza to build a Sforza family mausoleum.

The painting was commissioned by Sforza to be the centerpiece of the mausoleum.[5] The lunettes above the main painting, formed by the triple arched ceiling of the refectory, are painted with Sforza coats-of-arms.

The opposite wall of the refectory is covered by the Crucifixion fresco by Giovanni Donato da Montorfano, to which Leonardo added figures of the Sforza family in tempera. (These figures have deteriorated in much the same way as has The Last Supper.)

Leonardo began work on The Last Supper in 1495 and completed it in 1498—he did not work on the painting continuously. The beginning date is not certain, as the archives of the convent for the period have been destroyed, and a document dated 1497 indicates that the painting was nearly completed at that date.

According to wikipedia.