1 Corinthians 16:8

But I will tarry at Ephesus until Pentecost (KJV)

Then: The Corinthians are still hearing about Paul's original travel plan. Pentecost signified the beginning of the church in Jerusalem. Celebrated 50 days after Passover, people remembered the first coming of the Holy Spirit into the lives of 3000 individual church members (Baker, p. 242-3).

Now: We still remember Pentecost as part of the church calendar. Some churches give more emphasis to the memory than others. For some the focus is historical; for others the focus is on the specific need to be sensitive to the Lord's direction in an individual's life.

Extra Source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.