The Lord's Supper, the final meal of Jesus with his disciples when he is supposed to have instituted the communion celebration, is the subject of Leonardo Da Vinci's painting Last Supper. It also plays a key role in Dan Brown's conspiracy-driven religious mythology, but most readers of The Da Vinci Code don't seem to realize the degree to which Brown misrepresents the painting - perhaps due to their own religious and artistic illiteracy.
Leonardo Da Vinci was an artist and as such depended upon artistic conventions. The convention was for Judas to be seated opposite the others and with his back to the viewer; here Judas is seated on the same side of the table as the others. Another absent convention was to place halos over the heads of everyone but Judas. Leonardo's painting is thus more humanistic and less religious than most: Judas the betrayer is as much a part of the group as anyone, and everyone in the group is equally human rather than saintly and holy. This reflects Leonardo's humanistic and artistic beliefs, a strong mark against anyone trying to misuse the work in grand religious conspiracy theories.
We must also understand the scriptural sources of the Last Supper. Leonardo's immediate source is John 13:21, when Jesus announces that a disciple will betray him. It is also supposed to be a depiction of the origin of the communion ritual, but scripture is conflicted on what really happened. Only Corinthians is explicit in requiring that followers repeat the ritual, for example, and only only Matthew mentions that this is done for the forgiveness of sins.
These were not news reports: just as communion differs from one denomination to the next today, it differed among early Christian communities. Local customizing of religious rituals was normal and common, so what Da Vinci is portraying is his artistic interpretation of one community's localized communion liturgy, not a news report of historical events.
Dan Brown uses the scene for it's relationship to the Holy Grail, even though John doesn't mention bread or a cup. Brown somehow concludes that the absence of a cup means the Holy Grail must be something other than a cup: the disciple John, who is really Mary Magdalene. This is no more improbable than the orthodox Christian story, but it is an almost willful misrepresentation that is believed when people don't understand the artistic and religious sources.