Pentecost is both Jewish & Christian!

"The Jewish Harvest Festival, or Festival of Weeks, is also called Pentecost, a Greek word that means 'fifty' ... [It] shares the name Pentecost with the Christian commemoration of the coming of the Holy Spirit.  The Acts of the Apostles describes this event:  'When the day of Pentecost came, all the believers were gathered together in one place. ... They were all filled with the Holy Spirit' (2:1,4). 

"Acts is referring to the Harvest Festival, which takes place fifty days after Passover in the Jewish calendar.   Jesus was crucified near the end of Passover.  Fifty days after the Resurrection, Jews gathered in Jerusalem to celebrate the Harvest Festival - and then the Holy Spirit descended on Christ's followers. 

"That is why the Church commemorates Pentecost seven Sundays (or fifty days) after Easter Sunday."  

(Saint Mary's Press Parish Catalog Spring 2014 edition, p 44)