And when the day of Pentecost was fully come, they were all with one accord in one place. Acts 2:1
“The word “Pentecost” is a Greek word signifying the 50th part of a thing, or the 50th in order. Among the Jews it was a applied to one of their three great feasts which began on the 50th day after the Passover.” ¹
“Of this feast, which had its name from πεντηκοστη, pentecostee, (which signifies the fiftieth day,) because it was celebrated fifty days after the passover, see notes on Leviticus 23:15-16. As our Lord was crucified at one of the great Jewish feasts, it was fit that He should be glorified at another.”—Benson Commentary.
“There was an appearance of something like flaming fire, lighting on every one of them, according to John Baptist’s saying concerning Christ; He shall baptize you with the Holy Ghost, and with fire. The Spirit, like fire, melts the heart, burns up the dross, and kindles pious and devout affections in the soul; in which, as in the fire on the altar, the spiritual sacrifices are offered up. They were all filled with the Holy Ghost, more than before. They were filled with the graces of the Spirit, and more than ever under his sanctifying influences; more weaned from this world, and better acquainted with the other. They were more filled with the comforts of the Spirit, rejoiced more than ever in the love of Christ and the hope of heaven: in it all their griefs and fears were swallowed up. They were filled with the gifts of the Holy Ghost; they had miraculous powers for the furtherance of the gospel. They spake, not from previous though or meditation, but as the Spirit gave them utterance.”—Matthew Henry. More commentaries can be found here.