According to the ancient Christian tradition, the day after Pentecost or the Day of the Holy Trinity, which is always celebrated on a Sunday, the Orthodox Church glorifies the third person of the Holy Trinity - the Holy Spirit.
The feast of the Holy Spirit is celebrated on the Monday after Pentecost. This is connected with the tradition of the Eastern Church on the day after the great holiday to honor those persons who performed the main role in this holiday. On the day of Pentecost, they celebrate the very event of the descent of the Holy Spirit on the apostles, and on Monday they pay special honor to the Holy Spirit, as the third Person of God.