Leviticus 23:20
And the priest shall wave them with the bread of the firstfruits for a wave offering before the LORD, with the two lambs: they shall be holy to the LORD for the priest. And the priest shall wave them with the bread of the firstfruits for a wave offering before the LORD, with the two lambs: they shall be holy to the LORD for the priest.
Pentecost came fifty days after the offering of firstfruits and was also called the feast of harvest and the feast of weeks. The two loaves baked with leaven were presented before God as an expression of gratitude for grain prepared as food. The feast lasted only one day, which was devoted to religious service.