Holy Communion is one of the rites of Christianity. It is a memorial, a time when Christians remember the death and resurrection of Christ. It uses the fruit of the vine (juice or wine) and bread. The bread represents the body of Christ, and the juice represents the blood of Christ. When Christians eat and drink Holy Communion, they are represented by their communion with Jesus, and they are strengthened spiritually.

It was Christ himself who instituted Holy Communion in Matthew 26:26-28:

"While they were eating, Jesus took bread, and after blessing it, he broke it and gave it to his disciples, saying, 'Take, eat; this is my body.' And he took the cup and gave thanks, and gave it to them, saying, 'Drink from it, all of you. For this is my blood of the New Testament, which is shed for many for the forgiveness of sins."

In New Testament times, Holy Communion was different. It wasn't just bread and a cup of juice. It was a real meal, similar to Christmas or New Year's Eve. Paul even gave instructions on what the meal should be like. Nowadays, for economic reasons, they've kept only the basic elements (which doesn't affect the most important part, the spiritual one, in any way). As for the regularity of the meal, the Bible doesn't specify the period. Some churches celebrate it weekly, others monthly, quarterly, biannually, and annually. The Lord's Supper is part of the doctrine of Christ.