Acts 2:1-13
And when the day of Pentecost came [Note: This was a Jewish festival held fifty days following the Passover Festival. Lev. 23:15-21], they [i.e., the twelve apostles] were all gathered together in one place [i.e., their second story living quarters, 1:13 ?].
1 Corinthians 15:20
But now Christ has been raised up from the dead; [He is] the firstfruits of those who have died. [Note: By "firstfruits" is meant the first installment of the crop which promises the entire harvest eventually].
James 1:18
It was God's purpose to bring us into being [spiritually] by the message of truth, so that we could become a kind of "firstfruits" of those He created. [Note: "Firstfruits" here suggests either that Christians are the best of all of God's creatures (Num. 18:12), or that they were the first to be converted, with many more to come].