“When the day of Pentecost came”
When the day of Pentecost came, they were all together in one place. (Acts 2:1 BSB)
What it sounds like today
'Pentecost' now names a Christian event and a denomination; the underlying Jewish festival is invisible.
The cultural key
Pentecost is simply Greek for 'fiftieth': the Feast of Weeks (Shavuot), fifty days after Passover, when the wheat harvest's firstfruits were brought (Leviticus 23:15–21). It was one of the three pilgrimage festivals, which is why Jerusalem is packed with Jews 'from every nation under heaven' for Luke's nation list. Later Jewish tradition also celebrated it as the anniversary of the giving of the Law at Sinai (how early that link was made is debated), and readers have long heard Sinai echoes in the scene: wind, fire, and God's word going out in many tongues. The mockers' 'they are filled with new wine' earns Peter's dry reply that it is only nine in the morning.
Recovered meaning
The Spirit arrives on the harvest festival, and the first harvest is three thousand people. Luke's timing is theology: firstfruits on the day of firstfruits, and a word at Sinai's feast that every pilgrim hears in his own language.