Pentecost is one of the great festivals of the Church. In modern popular culture it isn't nearly as well known as Christmas or Easter, but for Christians it's profoundly significant. It marks the coming of the Holy Spirit on the disciples and their transformation from frightened and confused people to men who would face martyrdom for what they believed.
Pentecost is from the Greek word 'Pentekostos', which means 'fifty'. It's the 50th day after the Sabbath of Passover week and in Judaism is called the Feast of Weeks (Leviticus 23:16).
The word Pentecost has become more common in the UK, but traditionally the Church celebrated it as Whitsun – 'White Sunday'. It's believed that this is because it was a day for baptisms and those being baptised would wear white. Pentecost was when God poured out the wisdom of the Spirit.
Pentecost is the birthday of the Church.
It marks the beginning of Christianity as a purposeful movement and a new community. When Peter preached immediately afterwards, around 3,000 people believed and were baptised.