Noun
Pentecostarion (plural Pentecostaria)
(Christianity) A liturgical book for the Paschal season, used by the Eastern Orthodox and Eastern Catholic Churches that follow the Byzantine Rite.
The liturgical season from Easter to the Sunday of All Saints (the Sunday after Pentecost ) is known as the Pentecostarion (the "fifty days"). Source: Internet
Then the Pentecostarion will begin. Source: Internet