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 was being completed (in 1498). Leonardo's patron, Ludovico Sforza, planned that the church should be remodeled as a family mausoleum, and to this end, changes were made, perhaps to plans by Bramante. The entire plan was not carried out. and a smaller mortuary chapel was constructed, adjacent to the cloister.The painting was commissioned by Sforza to decorate the wall of the mausoleum. 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 worked on The Last Supper from about 1495 to 1498, but did not work continuously. The beginning date is not certain, as the archives of the convent for the period have been destroyed. A document dated 1497 indicates that the painting was nearly completed at that date. One story goes that a prior from the monastery complained to Leonardo about its delay, enraging him. He wrote to the head of the monastery, explaining he had been struggling to find the perfect villainous face for Judas, and that if he could not find a face corresponding with what he had in mind, he would use the features of the prior who had complained. (wikipedia)