Exodus 34:22
And thou shalt observe the feast of weeks, of the firstfruits of wheat harvest, and the feast of ingathering at the year’s end. And thou shalt observe the feast of weeks, of the firstfruits of wheat harvest, and the feast of ingathering at the year’s end.
The command was given during Moses's second forty days and nights alone with God on the mount. No one was permitted to go up with him or to approach the mount, and he was miraculously sustained through that time. He took with him two tables of stone that he had prepared at God's command, and God wrote upon them the words of the covenant, the Ten Commandments. When he descended his face shone with dazzling light reflecting the glory of the divine presence, so that Aaron and the people were afraid to come near him.
The feast of weeks is Pentecost, also called the feast of harvest, coming fifty days after the offering of firstfruits. It occupied only one day devoted to religious service. As an expression of gratitude for grain prepared as food, two loaves baked with leaven were presented before God at that feast.