What feast does Paul refer to in 1 Corinthians 5:8? What feast is Paul referring to?


  Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth. (1 Corinthians 5:8 NKJV)
The preceding verses answers your question:

6 Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump?
7Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:
8Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.

Verses 6 and 7 are referring to the Passover Feast. During Passover, Jews at the time (and today) would eat unleavened bread, as a symbol of moral purity.  Here Paul is explaining this symbolism.  In the analogy, leaven (yeast) represents sin, and he says that just as even a little yeast is sufficient to leaven a large loaf of bread, just a little sin (or sexual impurity in this specific example) is enough to contaminate an entire body of believers.