Shavuot, a Jewish holiday we call Pentecost, began this evening. Shavuot is a two-day holiday, celebrated from sunset on May 28 until nightfall on May 30. It coincides with the date that God gave the 613 commandments of the Law to the Jewish people at Mount Sinai more than 3,300 years ago.

It is celebrated with a festive meal, then staying up all night and learning Torah. The following day (Friday), they go to hear the Ten Commandments. The earliest source for this custom of all-night study writes, “this practice was a preparation for and in honor of the “bride’s” (the Jews) upcoming marriage to the “groom” (G‑d, or the Torah).” [from Chabad dot org]

The Jews were commanded in the Old Testament, "Three times in a year shall all thy males appear before the LORD thy God in the place which he shall choose; in the feast of unleavened bread, and in the feast of weeks, and in the feast of tabernacles: and they shall not appear before the LORD empty: Every man shall give as he is able, according to the blessing of the LORD thy God which he hath given thee." - Deuteronomy 16:16-17 KJV (The “feast of weeks” is Pentecost).

This is why, when the church began, with about an hundred and twenty in the upper room in Jerusalem, "And when the day of Pentecost was fully come, they were all with one accord in one place...there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven." - Acts 2:1, 5 KJV

This is how there were witnesses that understood the tongues of the disciples that had just been filled with the Holy Ghost. Each Jew, devout men, out of every nation, were there for Shavuot (Pentecost). Each one was able to recognize at least one of the disciples speaking in the language of his nation. It was a sign to them all that God was doing something miraculous.