Pentecost followed by Trinity Sunday
Liturgical celebration
Eastern churches
In the Eastern Orthodox Church, Pentecost is one of the Orthodox Great Feasts and is considered to be the highest ranking Great Feast of the Lord, second in rank only to Pascha (Easter). The service is celebrated with an All-night Vigil on the eve of the feast day, and the Divine Liturgy on the day of the feast itself. Orthodox churches are often decorated with greenery and flowers on this feast day, and the celebration is intentionally similar to the Jewish holiday of Shavuot, which celebrates the giving of the Mosaic Law. In the Coptic Orthodox Church of Alexandria, Pentecost is one of the seven Major "Lord's Feasts". 
The feast itself lasts three days. The first day is known as "Trinity Sunday"; the second day is known as "Spirit Monday" (or "Monday of the Holy Spirit"); and the third day, Tuesday, is called the "Third Day of the Trinity." The Afterfeast of Pentecost lasts for one week, during which fasting is not permitted, even on Wednesday and Friday. In the Orthodox Tradition, the liturgical color used at Pentecost is green, and the clergy and faithful carry flowers and green branches in their hands during the services.
All of the remaining days of the ecclesiastical year, until the preparation for the next Great Lent, are named for the day after Pentecost on which they occur (for example, the 13th Tuesday after Pentecost). (There is one exception: for some reason, the Melkite Greek Catholic Church begins marking Sundays "after Holy Cross".)
The Orthodox icon of the feast depicts the Twelve Apostles seated in a semicircle (sometimes the Theotokos (Virgin Mary) is shown sitting in the center of them). At the top of the icon, the Holy Spirit, in the form of tongues of fire, is descending upon them. At the bottom is an allegorical figure, called Kosmos, which symbolizes the world. Although Kosmos is crowned with earthly glory he sits in the darkness caused by the ignorance of God. He is holding a towel on which have been placed 12 scrolls, representing the teaching of the Twelve Apostles.