The church season we begin on Pentecost Sunday is the longest of the year - running from now through the summer and fall until Advent begins at the end of November.  And while Pentecost itself focuses on the dramatic gift of the Holy Spirit to the early church, with images of wind and flame (and the color red), the season of Pentecost is different.  
The season is long, and calm (no major holidays and lots of folks in and out over the summer), with the liturgical color being green, and the focus being on slow growth.  
The Holy Spirit is most certainly present, but in a quieter and more subtle way.  It's my favorite season of the church year.