Pentecost - the pouring out of God's Holy Spirit at the Festival of first Fruits (Shavuot), in Jerusalem - is the day which God confirms Christ as Lord in and through the Apostle's preaching.  He (God) takes a legal obligation of Moses, to give thanks for the first fruits of harvest, and make is a celebration of the first fruits unto eternal through the resurrection of Christ Jesus.  We are, through Christ, the harvest brought in to give glory to God, through the power of the Holy Spirit as God's Word of the Good News is preached to all.  How is Pentecost a real ongoing happening even today?