When the most High came down and confused the tongues, He divided the nations; But when he distributed the tongues of fire He called all to unity. Therefore, with one voice, we glorify the All-holy Spirit! So goes the Kontakion of the Feast of Pentecost.
On Pentecost we celebrate the decent of the Holy Spirit on the Apostles in the upper room. Fifty days following Passover the followers of Jesus were waiting in the upper room. Orthodox Theologian Protopresbyter Father Alexander Schmemann commented the significance of the fifty days.
"Pentecost in Greek means fifty, and in the sacred biblical symbolism of numbers, the number fifty symbolizes both the fulness of time and that which is beyond time: the Kingdom of God itself. It symbolizes the fulness of time by its first component: 49, which is the fulness of seven (7 x 7): the number of time. And, it symbolizes that which is beyond time by its second component: 49 + 1, this one being the new day, the "day without evening" of God's eternal Kingdom. With the descent of the Holy Spirit upon Christ's disciples, the time of salvation, the Divine work of redemption has been completed, the fulness revealed, all gifts bestowed: it belongs to us now to "appropriate" these gifts, to be that which we have become in Christ: participants and citizens of His Kingdom." Fr. Alexander goes on to explain more about the feast of Pentecost.