On what day is the Last Supper celebrated?

It was the last time that Jesus spoke with his disciples before the Crucifixion, and he showed them how to consecrate the Eucarist (sic). (by Lordcatz8)

Actually, all he did was ask his disciples to remember what he was about to do. He took the bread and said, "Take, eat: this is my body, which is broken for you: do this in remembrance of me." The Catholics take that literally and say "hoc est enim corpus meum," (Latin for, "for this is my body.") which they believe turns the bread into the actual body of Christ.

The rest of us see the broken bread as a representation of his broken body and remember what he did for us when we have communion.