First, Pentecost (aka Shavuot or the Feast of Weeks) is one of the 3 annual pilgrimage festivals of the Jewish year. Observance of Shavuot is included in the festival calendars in the Torah. Every adult Jew was expected to travel to Jerusalem on each of these festivals. The story of Pentecost in Acts occurs as the disciples are taking part in this pilgrimage, so they were already celebrating it before the events remembered in Acts. Prior to the story in Acts, it's not clear that the early Christians saw their mission as including preaching to non-Jews. After this event, preaching to non-Jews seems to be a natural consequence of the idea that each listener heard the message in their own language.