When the Day of Pentecost had fully come, they were all with one accord in one place. 2 And suddenly there came a sound from heaven, as of a rushing mighty wind, and it filled the whole house where they were sitting. 3 Then there appeared to them divided tongues, as of fire, and one sat upon each of them. 4 And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance. — Acts 2:1-4.
The Day of Pentecost comes fifty days after the Sabbath in Passover week. When this historical Lord’s day (first day of the week) arrives, God sends the Holy Spirit in an incredible way as the followers of Jesus are praying together. He appears as ‘divided tongues, as of fire’. Each person is filled with the Holy Spirit and begins to speak with other tongues (languages not known previously), as the Spirit enables them.