The name "Pentecost" derives from a Greek word meaning "fiftieth." The reason is that the date falls 50 days after Easter, the highest holy day on the Christian calendar.
For the original Jewish festival, the significance was that it was 50 days after Passover. Regardless, 50 is a prominent calendar feature.
"This name came into use in the late Old Testament period and was inherited by the authors of the New Testament," noted the National Catholic Register.
The Pentecostal Movement derives its name from the holy day, as their worship stresses the receiving and the moving of the Holy Spirit.