Pentecost/Trinity Sunday

A book of Orthodox feast-day traditions characterizes today, the 50th day after Holy Pascha, as follows (emphasis added):

On the day corresponding to this day of salvation, the day of Pentecost, the Savior's Apostles, who were suddenly instructed by the coming of the Holy Spirit, became possessors of the greatest wisdom and spoke clearly about heavenly doctrines. They became preachers of Truth and teachers to the whole world. From that day they began the work of their great mission, the wonderful and delectable first-fruit of which was the conversion of 3,000 souls on that very same day.