Commentaries:
While the Passover is an annual observance—happening just once each year—the issue here is the congregation occasionally gathering as a church for a meal. The text implies that their "com[ing] together as a church" was a frequent occurrence rather than an annual one. The event he speaks of, then, was not the Passover, despite his mentioning it in the midst of his admonition. The congregational gathering was an occasion for food, fellowship, and fun—perhaps akin to the "love feasts" of Jude 12.
Yet we can tell that Paul took great exception to what occurred during those feasts because his admonition is quite stern. He describes blatant self-centeredness, lack of self-control, and general despising of God's church through some shaming and despising others on account of their personal situations.
Following this, in verses 23-26, the apostle brings up the Passover for a couple of reasons. First, some were confused, thinking that any church gathering was a time to commemorate the Passover. Thus, Paul had told them in verse 20 that when they came together, it was not to eat the Lord's Supper. He feels a need to give them a brief refresher on what the Passover is.
Second, the Passover is a powerful, motivational teaching tool to get his point across about their shortcomings. So, in verse 27, he underscores the matter's seriousness: "Therefore whoever eats this bread or drinks this cup of the Lord in an unworthy manner will be guilty of the body and blood of the Lord." We must understand this is within the context of their behavior in the congregation, where some were only looking out for their own interests, and others were being despised and shamed. That context does not end until the chapter's end in verse 34.
The apostle is warning them about taking the Passover while they continued to perpetrate this social and spiritual violence against their brethren. Observing the Passover in such an unworthy way will make an offender guilty of Christ's body and blood. This matter of worthiness is so crucial that he instructs church members to perform an examination—an evaluation—of our spiritual state before we partake of the Passover: "But let a man examine himself, and so let him eat of the bread and drink of the cup" (I Corinthians 11:28). God will not look favorably on those who mistreat the ones for whom Christ died.
David C. Grabbe
What Does 'Discerning the Lord's Body' Mean? (Part One)
Other Forerunner Commentary entries containing 1 Corinthians 11:20:
Exodus 12:8
1 Corinthians 11:23-29
Jude 1:12