The Jewish Feast of Shavuot (Pentecost in Greek) is the anniversary marking the first covenant. the giving of the Torah to the Jews on Mount Sinai, it is also a harvest festival.
All the disciples had gathered in Jerusalem. They met in the upper room where they had held the Last Supper and where Jesus had appeared to them in resurrected form. Suddenly the Holy Spirit descemded amd fell on them all and was poured out upon them with tongues of fire appearing over their heads. A supernatural occurance that was to change them for all time. Today we celebrate the coming of the Holy Spirit who resides with us still.