To celebrate a festival, to keep a feast — the act of communal, joyful observance. In the NT, Paul transforms this from Old Covenant ritual language into a description of the entire Christian life.
Paul's command to 'celebrate the festival' (1 Cor 5:8) is revolutionary. The old leaven of malice must be purged, and believers must feast on 'sincerity and truth.' For Paul, the Christian life IS the feast — not once a year, but perpetually. Christ our Passover has been sacrificed, so every day is a celebration of deliverance. This transforms ethics from duty into delight.