What is the Christian Holiday of Pentecost? Shutterstock.com Pentecost—also known as “Whitsunday,” “White Sunday” or “Trinity Sunday”—is a Christian holiday that commemorates the outpouring of the Holy Spirit on the apostles and followers of Jesus (shortly after Christ’s ascension), as described in Acts chapter 2. The holiday falls on the seventh Sunday after Easter, which is roughly fifty days after the commemoration of that most important of Christian holy days. Hence the name “Pentecost”—which comes from the Greek word πεντηκοστή, meaning quite literally “fiftieth” or the fiftieth day. Unlike Christmas, which is a “set date” holiday (always falling on the 25th of December), Pentecost’s commemoration is determined by the date of Easter and, since Easter falls on a different date each year, Pentecost is a “movable” Solemnity (Roman Catholic), Great Feast (Eastern Orthodox), Principal Feast (Anglicanism) or Festival (Lutheranism).