Pentecost for Western Christians is also known as Whitsunday in some Western churches. Pentecost is the commemoration of the coming of the Holy Spirit upon the disciples of Jesus following His Ascention (Acts 2: 1-11). It comes fifty days after Easter.
Ascension (17th for Eastern Churches, 24th for Western Churches) is the anniversary of Jesus' ascension into heaven and enthronement as universal sovereign. It comes 40 days after Easter/Pasha.
Easter or Pascha (Christianity)
Easter (Western) or Pascha (Eastern) celebrates the resurrection of Jesus. It initiates the fifty-day period culminating in Pentecost.
Lazarus Saturday (Eastern Christianity)
Lazarus Saturday commemorates the raising of Lazarus from the dead by Jesus (John 11:1-45).