Pentecost takes its name from a Greek expression meaning "the fiftieth day" and is also known by names such as Whitsun or Whit Sunday in the United Kingdom and other English-speaking places. It is observed as both a holiday and a season within the Christian liturgical calendar; the term and its usage originate in Greek tradition.

When it occurs

Pentecost is celebrated on the fiftieth day following Easter. It also falls ten days after Ascension Thursday and therefore its date varies from year to year according to the dates of Easter.

Why it matters

The observance commemorates the coming of the Holy Spirit upon the gathered Apostles and other followers of Jesus. This event is described in the book of Acts in the Bible, and is connected with promises made earlier by Jesus after his resurrection. Early accounts report that about 3,000 people were baptized that day, a development many traditions regard as marking the beginnings of the Christian Church.