On the tradition of decorating the church with greenery on the feast of the Holy Trinity.

In Russia, Holy Pentecost was celebrated not in the first years after the baptism of Rus, but almost 300 years later, in the 14th century, with St. Sergei of Radonezh.
Holy Trinity is a very beautiful feast. Orthodox Christians on this day decorate houses and temples with green branches and flowers. This custom goes back to the Old Testament, when houses and synagogues were decorated with greens at Pentecost in memory of how, under the Sinai Mountains, everything bloomed and green on the day when Moses received the tablets of the law. The upper chamber, where the Holy Spirit fell upon the apostles, at that time, according to common custom, was also decorated with branches of trees and flowers. On the feast of the Trinity, the phenomenon of Abraham the Trinity is remembered in the Mamvrii oak, so the church decorated with greenery reminds us of that oak forest. And the flowering branches remind us that under the influence of God's grace human souls blossom with the fruits of virtues.
Houses and temples are decorated with branches, grass, flowers. Greenery, flowers symbolize life. So people express joy and gratitude to God for having restored them through baptism into a new life. Touching and beautiful is the Divine service on the feast of the Holy Pentecost. The church is decorated, the priests are dressed in green garments, the grass smells of fresh greenery, the choir "solemnly and lightly sounds": "renew in our hearts, the Almighty, the true, right Spirit", the special prayers of St. Basil the Great are knelt. And in the courtyard of an early summer - a reminder of the beautiful and deep "summer of the Lord", which Jesus Christ promised the righteous.
The color of the Trinity feast is emerald green. The emerald cloud shines the temples from the inside - hundreds of birch twigs are carried by parishioners, the floor in the church is densely covered with grass, the smell of June is intensified by the rays of the sun from church windows, mixed with subtle notes of incense and wax candles. In the Orthodox world it is customary for the Trinity to decorate churches and dwellings with branches of young plants, often birch trees, which indicates the renewal of all things by the power of the Descended Holy Spirit, to the flowering of all vital forces.
Historically, birch branches have been used to decorate churches and houses. This tree is considered blessed in Russia. There are many verses and songs devoted to it. The Trinity feast without birch is the same as Christmas without a Christmas tree. But Russia is a big country, with different climatic conditions, apparently, this fact can be explained by the fact that in some places the trees were oak, maple, mountain ash.
Consecrated Trinity grass can be used for sachet - small pillows with aromatic herbs, which are placed with linens in cabinets, and can also be placed at the head of the bed. Also the Trinity grass can also be used to make a pillow "for death" - every Christian should be ready for the hour of death. Also, the Trinity grass is used for the manger in the Nativity cave.
Korinfsky A.A. The Trinity is the Green Christmas Tree.