"You shall keep the Feast of Unleavened Bread. Seven days you shall eat unleavened bread, as I commanded you, at the time appointed in the month Abib, for in the month Abib you came out from Egypt. All that open the womb are mine, all your male livestock, the firstborn of cow and sheep. The firstborn of a donkey you shall redeem with a lamb, or if you will not redeem it you shall break its neck. All the firstborn of your sons you shall redeem. And none shall appear before me empty-handed. "Six days you shall work, but on the seventh day you shall rest. In plowing time and in harvest you shall rest. You shall observe the Feast of Weeks, the firstfruits of wheat harvest, and the Feast of Ingathering at the year's end. Three times in the year shall all your males appear before the LORD God, the God of Israel. For I will cast out nations before you and enlarge your borders; no one shall covet your land, when you go up to appear before the LORD your God three times in the year. "You shall not offer the blood of my sacrifice with anything leavened, or let the sacrifice of the Feast of the Passover remain until the morning. The best of the firstfruits of your ground you shall bring to the house of the LORD your God. You shall not boil a young goat in its mother's milk."
Exodus 34:18-26
The commands concerning the feast of unleavened bread (18), the dedication of the firstborn (19, 20) and the other feasts (22, 23) had already been given to Israel (Exodus 12:14-20, 13:1-19) and the reason for their repetition here is that, the people having been reinstated in the Lord's favour through the intercession of Moses, He now proceeded to re-lay, as it were, the foundations of their religious life. That none should appear before God empty (20) was meant to signify that the offerings brought to Him were the token of their total submission to Him, their witness to the fact that they were not their own. There is an important principle enshrined in this which is of wide application, and it is seen, for example, in the laws concerning tithing. The people were to give a tithe as a recognition that all they had belonged to God. The tithe is a symbol, a token: it proclaimed that they were God's own possession.