The ancient Jewish Feast of Weeks (Lev 23:15-21) falls forty-nine days after the Jewish Passover. An offering of thanksgiving was appointed for the day after these seven weeks, i.e. the fiftieth day; the Greek word for “fiftieth” is Pentekostes.
In Palestine the grain harvest lasted seven weeks and was a season of gladness (Jer. 5:24; Deut. 16:9; Isa. 9:2). It began with the harvesting of the barley (Men. 65-66) during the Passover and ended with the harvesting of the wheat at Pentecost, the wheat being the last cereal to ripen”.(Jewish Encyclopedia on “Pentecost”)
It is at this feast, ten days after the Ascension of the Lord and seven weeks after his crucifixion at Passover, that the Holy Spirit came on all the believers (Acts 2). Pious Jews who had attended the Paschal feast had now returned to Jerusalem from their homes all over the known world, and they saw the very public birth of the New Testament Church.
The Pentecostarion is the service book of the Orthodox Church that provides the texts for the moveable portions of the divine services from Pascha through the feast of All Saints (the Sunday following Pentecost).
Forty days after Pascha
The Ascension of Christ
Sixth Sunday after Pascha
Fathers of the First Ecumenical Council
Seventh Sunday after Pascha
First Sunday after Pentecost
Feast of All Saints
Second Sunday after Pentecost
All Saints of Russia