Ascension Thursday is the day 40 days after Easter when Christ ascended to Heaven. See the Acts of the Apostles, Chapter 1:
Pentecost, in the second Chapter of Acts immediately after, was when the Holy Spirit descended on the Apostles. It is both the final revelation of the Holy Trinity and the foundation of the Christian Church.
Pente = 50
Pentecost is the 50th day after Pascha (Easter). As Pascha is linked with the Jewish Passover so the Christian Pentecost is linked with Jewish Pentecost, the feast for which Jews of many languages had come to Jerusalem (Acts 2)