The event in the above verses describes what has commonly become known as the "birthday of the church" - the coming of the Holy Spirit, after the sacrifice of Jesus Christ, on a much more widely available basis than it had been in the Old Testament era.
Pentecost originated after the Exodus when it was variously called the Feast of First Fruits, the Feast of Weeks, or the Feast of Harvest (Leviticus 23:15-21, Deuteronomy 16:9-12). It was observed 50 days after the ceremonial cutting of the first grain offering after the Passover - hence the origin of the Greek word Pentecost, which means to "fifty count."
The Old Testament Feast of First Fruits occurred 50 days after the slaying of the Passover lamb. By no coincidence, the New Testament Pentecost, as we read in the opening verses, occurred 50 days after The Crucifixion of Jesus Christ, the "Lamb of God."
All of the Biblical Holy Days served a dual purpose, the earlier as symbols for what was to come later. Their observance, and fulfillment, occupy much of Bible History and Prophecy:
The Dual Meaning Of The God-Commanded Biblical Holy Days
The spring Holy Days symbolize the events related to the First Coming of Jesus Christ:
The autumn Holy Days symbolize the events related to the Second Coming of Jesus Christ:
The "First Harvest" Of Salvation
The Old Testament festival of First Fruits celebrated the first harvest of the grain crop, while it actually symbolized the New Testament "first harvest" of humanity, the "first fruits of salvation":
"But in fact Christ has been raised from the dead, the first fruits of those who have fallen asleep. For as by a man came death, by a man has come also the resurrection of the dead. For as in Adam all die, so also in Christ shall all be made alive. But each in his own order: Christ the first fruits, then at His coming those who belong to Christ." (1 Corinthians 15:20-23 RSV) (see The Return Of Jesus Christ)
The first fruits were chosen for a purpose, not as any sort of favoritism. They will serve and teach, not just the unconverted physical people who will be alive at the Return of Christ, but those of the great resurrection (which will be vastly larger than the first) that will occur after the 1,000 years:
"The rest of the dead did not come to life until the thousand years were ended." (Revelation 20:5 RSV)
"your teacher will not hide himself any more, but your eyes shall see your teacher. And your ears shall hear a word behind you, saying, "This is the way, walk in it," when you turn to the right or when you turn to the left." (Isaiah 30:20-21 RSV)
The Holy Bible, from the beginning of Genesis to the end of Revelation, speaks of God's Plan of Salvation - a plan with a definite element of timing. The simple reality, as expressed so often through your Bible, is that not everyone is called to repentance, is able to repent even when they hear the words of the Gospel, at the same time. There is a definite order of calling, which is the reason that Jesus Christ often used the term "He who has ears to hear, let him hear." (Matthew 11:15 RSV).
Fact Finder: Did James refer to the "first wave" of Christians as the "first fruits of creation"?