Psalms 81:4

“For this was a statute for Israel, etc.] It was not a piece of will worship, or device of the children of Israel, but was of divine institution; that the passover should be kept at the time it was; and that the trumpets should be blown on the new moon, or first of Tisri; and that the feast of tabernacles should be kept on the fifteenth of the same month: and a law of the God of Jacob; and therefore to be observed by Jacob’s posterity: the law for the one is in (<021218>Exodus 12:18,19) and for the other is in (<032324>Leviticus 23:24,34) and so all the ordinances of Christ, and of the Gospel dispensation, are to be regarded on the same account, because they are the statutes and appointments…”

“For this was a statute for Israel ... - See Exodus 12:3. That is, it was a law for the whole Jewish people, for all who had the name Israel, for all the descendants of Jacob. The word was is not in the original, as if this had been an old commandment which might now be obsolete, but the idea is one of perpetuity: it is a perpetual law for the Hebrew people. A law of the God of Jacob - Hebrew, a judgment; or, right. The idea is, that it was what was due to God; what was his right. It was a solemn claim that he should be thus acknowledged. It was not a matter of conventional arrangement, or a matter of convenience to them; nor was it to be observed merely because it was found to be expedient…”

“81:4-5 The festival, a time to celebrate and listen to the Lord, took place at God’s appointed time.”

“Psa 81:4-5 now tell whence the feast which is to be met with singing and music has acquired such a high significance: it is a divine institution coming from the time of the redemption by the hand of Moses. It is called חק as being a legally sanctioned decree, משׁפּט as being a lawfully binding appointment, and עדוּת as being a positive declaration of the divine will. The ל in לישׂראל characterizes Israel as the receiver, in לאלהי the God of Israel as the owner, i.e., Author and Lawgiver. By בּצעתו the establishing of the statute is dated back to the time of the Exodus; but the statement of the time of its being established, "when He went out over the land of Egypt," cannot be understood of…”