The commentaries are about equally divided as to which festival or festivals are intended here. Is it speaking of the spring or the fall feasts? Passover and Tabernacles both fall on a full moon, and obviously, both are preceded on the first day of the month by a new moon. However, two things tend to throw the weight toward the fall festivals.
First, the superscription of the psalm is gittith. Gittith, though it is the term for a musical instrument, literally means "winepress." It is associated with the fall harvest of grapes because of its customary use by the vintners then.
Second, and more authoritative, is the word "trumpet." All the commentators duly report that this refers to the ram's horn. Instructions for the use of trumpets are given in Numbers 10, but it makes no distinction as to which trumpet (ram's horn or silver) should sound at the beginning of each month. However, Jewish tradition emphasizes that the silver horns were blown at the beginning of each month except Tishri 1, the Feast of Trumpets. The weight of evidence at this point inclines toward the feasts of Trumpets and Tabernacles.
Verse 4 explains that God established a law in Joseph as a testimony witnessing to a historical fact: his release from prison and elevation to prime minister. Thus, this psalm gives us the time setting for Joseph's experiences in Genesis 41: Tishri 1, which became the Feast of Trumpets! On this feast day, Joseph was removed from his hard labor, as Psalm 81:6 relates. The psalmist shows an encouraging parallel between God and Joseph. Both are saviors, and we can take encouragement from what happened to Joseph. God did not forget him!
John W. Ritenbaugh
God's Promises Are Sure!
We need to be aware of a danger inherent in festival times: that our pursuit of joy does not obscure more important elements. Psalm 81 is a festival psalm, and these verses bid us to enjoy God's feasts fully.
God commands us to rejoice in His feasts (Deuteronomy 14:26), but Psalm 81:8-10 cautions us to remember certain things so that their real purpose is not lost in an unthinking keeping of that command:
Hear, O My people, and I will admonish you! O Israel, if you will listen to Me! There shall be no foreign god among you; nor shall you worship any foreign god. I am the LORD your God, who brought you out of the land of Egypt; open your mouth wide, and I will fill it.
God knows that even among His people, human frailty can misuse festival occasions, for they seem to beckon us to play. Relaxation and merriment tend to become the sole interest. Yet the greater the gaiety, the more obscure God's intent for the feasts become, and their spiritual value diminishes. God reminds us of the meaning of our songs of praise lest our joy becomes gaiety, gaiety becomes hilarity, hilarity becomes revelry, and revelry becomes debauchery. Our God-produced joy is lost.
"Listen to Me while you rejoice," God says. "Stay completely clear of idolatry and remember I am the God who freed you from your bondage. Open your mouth and I will feed you!" When we follow God's prescription, He will feed us so that we experience real joy and satisfaction. God removes the burdens that make true rejoicing a reality. He continues, "I would feed you with the finest of wheat, and with honey from the rock I would satisfy you" (verse 16). He makes it plain that real joy lies in the quality of our relationship with Him!
John W. Ritenbaugh
The Fruit of the Spirit: Joy