>Today is Holy Spirit Day—the day after the Feast of Pentecost. He is the Life Creating Spirit, the Comforter, the Spirit of Truth. He is One of the Holy Trinity, the One that Christ promised to send when he ascended to Heaven (as we celebrated on the Feast of Ascension, on June 5.) 
This is the icon of the Feast of Pentecost in the nave at St. John (right).
During the season of Pascha (the time from Pascha to Pentecost) we don’t pray the “O Heavenly King” prayer in the Orthodox Church, because we’re remembering the time when Jesus was still with his disciples and the others on earth. While he was with them, they didn’t yet need the presence of the Holy Spirit as a Helper. But once he ascended to Heaven to be with the Father, they needed the Third Person of the Trinity to abide with them forever. To comfort them.
We need the Holy Spirit in our lives, too. So, yesterday morning at the great Feast of Pentecost (at my parish here in Memphis, St. John Orthodox Church) and again last night at “Kneeling Vespers,” we prayed the prayer that hasn’t been part of our services since Pascha:
O Heavenly King, O Comforter,
The Spirit of Truth,
Who art in all places and fillest all things;
The treasury of good things and giver of life,
Come and abide in us,
Cleanse us from every stain
And save our souls, O Good One.
We haven’t knelt during services since before Pascha, either, so kneeling vespers re-instituted this practice, which we will return to now, in “normal time.” I say normal time because we are no longer in the Great Fast or the Great Feast, but in a season between the major fasts and feasts. Oh, sure, we’ll have the Fast and Feast of the Apostles the last week of June this year. And the Fast and Feast of the Dormition of the Mother of God in August. But these are relatively short periods outside “normal time.” I love the rhythm and balance that the Orthodox calendar brings to our lives.
We decorated the nave of our temple with greenery for Pentecost, because green is the color of renewal, of new birth, of life.
Our clergy wore green robes (the ones who have green robes) and the altar and icon stands were adorned with green cloths.
As I breathed in the scent of the live greenery 
and watched the sun’s rays hitting the plants, the icons, and the embroidered robes of the clergy, I was struck, again, by how organic the Church is. It’s always been GREEN. We bake holy bread from the fruit of the earth and offer it back to Christ, and it becomes the Body and Blood of our Lord in a mystery. We take organic materials like wood and pigments and eggs and we turn them into icons to adorn our temples and our homes to remind us that God became Man to redeem us. We take beeswax and make candles and light them in front of the icons to remind us that the Light of Christ illumines us. (Read a short and wonderful piece by St. Nicolai Velmirovich about why we light candles here .)
Father Justin Patterson writes about one of his experiences with Pentecost while visiting Russia in his blog:
The Feast of Pentecost remains one of the most beloved feasts in Orthodox Christian piety and experience. I’ll never forget my “Pentecostal” experience in Russia. As I walked into the church for the Pentecost Kneeling Vespers on Sunday afternoon, the smell of freshly cut greenery overwhelmed my senses. I waded through cut grass that was at least a foot deep, spread out on the floor of the nave. Along the sides of the walls, dozens of small trees had been brought into the church, in full blossom. Life was in the air! Together, we Orthodox believers—crammed into that little church, were entering into the reality of the outpouring of the Holy Spirit upon the disciples.
So, Happy Holy Spirit Day! As I begin a new work week, with “to do” lists that include mundane things like paying bills and running errands and the creative work of writing and revising my memoir-in-progress, I’m thankful for the comfort and presence of the Third Person of the Trinity. I know I will forget Him over and over, and I’ll even sin against Him before the sun sets today, but He will not forget me. And because He is the Spirit of Truth, He will help me seek truth and write truth as I work.
O Heavenly King, O Comforter, the Spirit of Truth…. Come and abide in us.