Such outward festivals and rites for the worship of God are established on account of the crude people, to keep them so busy that they will not invent festivals by their own efforts. Human nature tends unceasingly to set up ceremonies and institute forms for worshiping God. Therefore it is necessary that it be curbed and kept in the Word of God, through which we are sure that what we do is divinely instituted and pleases God. The festivals have this purpose too, that the people come together at least twice or three times a year, hear and learn the Law of God, and be kept in the unity of faith and life.
There are, however, three things which he wanted to be remembered at these three festivals. In the Festival of the Passover they should recall the Exodus from Egypt. This was the first of all the festivals and the chief one at the beginning of the year, because of the first and highest miracle shown them, namely, their liberation from Egypt. At the Pentecost Festival they should remember the receiving of the Law on Mt. Sinai, since, of course, the Law was given in the third month after the Exodus, almost fifty days after the Passover. At the Feast of Tabernacles they were to remember all the physical benefits shown in all those forty years in the desert. So you see that the festivals are established, not for the sake of luxury, leisure, or pleasure but for the sake of our salvation and the glory of God, that the Word of God may be heard and His blessings remembered, that we may be instructed, nourished, and preserved in faith and love.
Luther’s works, vol. 9: Lectures on Deuteronomy 1525