Leviticus 23:15

then make an offering by fire to Yahweh, a burnt offering or a sacrifice to fulfill a special vow, or as a freewill offering or in your appointed times, to make a soothing aroma to Yahweh, from the herd or from the flock.

‘Also on the day of the first fruits, when you bring near a new grain offering to Yahweh in your Feast of Weeks, you shall have a holy convocation; you shall do no laborious work.

“You shall count seven weeks for yourself; you shall begin to count seven weeks from the time you begin to put the sickle to the standing grain.

“And it shall be the prince’s part to provide the burnt offerings, the grain offerings, and the drink offerings, at the feasts, on the new moons, and on the sabbaths, at all the appointed times of the house of Israel; he shall provide the sin offering, the grain offering, the burnt offering, and the peace offerings, to make atonement for the house of Israel.”

And when the day of Pentecost had fully come, they were all together in one place.

The Feast of Weeks