The first fruits of the Holy Spirit are the true believers won to Elohim by His Son the firstborn. (1 Cor. 15:20&23, James 1:18, Rev. 14:4)
The ancient ceremony of presenting the Almighty with the firstfruits of the earth (Deut. 26:1-11) with two oven-baked loaves, in which the leaven had been de-activated (Lev. 23:15-17), was a figure, a type, a shadow of the spiritual harvest of human souls in whom the yeast of sin will one day cease to exist. There is coming a day when the power of sin will be de-activated in us believers just as the power of leaven is neutralised in an oven baked loaf.
The breathtaking experience recorded in Acts 2 was but the early harvest, a kind of "firstfruits" of an even greater out-pouring of divine power scheduled to fall upon the church in the near future when the main harvest of human beings will be gathered in.
The Feast of Pentecost, in short, prefigures, guarantees and commemorates the early harvest, the first fruits of human souls: and those who celebrate it declare their willingness to be part of that spiritual multitude which will one day be gathered in and presented to the Almighty by His Son - the Firstborn!