The descent of the Holy Spirit upon the Apostles fifty days after Pascha, marking the full revelation of the Holy Trinity and the beginning of the Church's mission in the world. During Vespers, special kneeling prayers are offered and the church is adorned with greenery. This feast celebrates the completion of Christ's saving work and the outpouring of the Holy Spirit upon all believers. From this day forward, the Sundays of the year are numbered from Pentecost, signifying the continual life and growth of the Church in the grace of the Spirit.

The central worship service of the Orthodox Church, celebrating the Eucharist (Holy Communion). Usually celebrated on Sundays and feast days, the Divine Liturgy consists of the Liturgy of the Word (scripture readings) and Liturgy of the Faithful (Eucharistic prayer and communion). The most common forms are the Liturgies of St. John Chrysostom and St. Basil the Great.

The day after Pentecost, continuing the celebration of the Holy Spirit's descent upon the Church. Also called 'Trinity Monday,' this feast honors the Third Person of the Trinity specifically. Many Orthodox churches hold special services with kneeling prayers, emphasizing the ongoing work of the Spirit in the Church and in believers' lives.

The Tuesday after Pentecost, continuing the celebration of the Trinity and the gift of the Holy Spirit. This day maintains the festal character of the Pentecost season with special hymns and prayers. Many Orthodox communities hold additional services during Trinity Week to honor the complete revelation of the Triune God.

The first Christian Emperor Constantine the Great and his mother, St. Helen, who found the True Cross. Celebrated on May 21st, this feast honors Constantine's legalization of Christianity and Helen's pilgrimage to the Holy Land. Their work established Christianity's place in the Byzantine Empire and preserved many holy sites.