מדרש אויף תהילים 81:1

To the conductor over the Gittith. This refers to what is said in the scripture (Joel 4:13) "Send forth the sickle, for the harvest is ripe." Rabbi Pinchas said in the name of Rabbi Chelkia, "To whom was it said, 'Send forth the sickle?' The angels said to God, and Rabbi said to Israel. We do not sing about the harvest or the vintage, but about the Gittith (as it is written) [Psalms 8:1], 'For the Gittith.' The harvest refers to Babylon (Jeremiah 51:33) 'The daughter of Babylon is like a threshing floor at the time when it is trodden.' The vintage refers to Media, as it is said (Zechariah 9:13) 'For I have bent Judah as my bow, I have filled the bow with Ephraim.' Redemption was found in the four languages during the harvest, in the vintage, and in childbirth. If a person passes over them at the wrong time, he loses them, and even their owners do not benefit from them at all. This is what is written (Joel 4:13) "Send forth the sickle, for the harvest is ripe." In the harvest (Jeremiah 49:9) "If grape-gatherers came to you." In childbirth (Micah 5:2) "Therefore will he give them up, until the time that she who travaileth hath brought forth." In the balsam (Song of Solomon 8:14) "On the mountains of spices." Similarly, it is said (Joel 4:13) "Come down; for the press is full, the fats overflow." Everyone sees the Gittith. Joel speaks about it as it is said (Joel 4:13) "Send forth the sickle." Isaiah speaks about it as it is said (Isaiah 27:2) "And it shall come to pass in that day, that a vineyard of wine, red and good, shall be planted." Asaph speaks about it as it is said (Psalms 81:1) "For the Gittith." David speaks about it as it is said, "To the conductor over the Gittith."

... Another explanation. Why does the verse (Psalms 81:2) mention Yaakov and not all of the forefathers? Our rabbis taught: a man is measured with the measure by which he measures, as it says “In that measure (b’saseah), when they sent them out, it strove with it…” (Isaiah 27:8) A seah with a seah. I only know seah, from where do I learn tirkav and a half tirkav, kab and a half kav, rovah and a half rovah, a tuman and an uchlah? Scripture says “For every victory shout (s’on soein) sounds with clamor…” (Isaiah 9:4) Here there are many measures. I only know about something which comes by measure, from where do I learn little bits that add up to a large amount? Scripture says “…adding one to another to find out the account.” (Ecclesiastes 7:27) This is like a king who had three people that loved him, and sought to build himself a palace. He brought the first one and said to him: I want to build a palace for myself. His love replied to him: I recall that it was a mountain in the beginning. He called to the second love and said to him: I want to build a palace for myself. He replied: I recall that it was a field in the beginning. He called to the third love and said: I want to build a palace for myself. He replied: I recall that it was a palace in the beginning. He said to him: by your life! I will build that palace and call it by your name. So too Avraham, Yitzchak and Yaakov were lovers of the Holy One. Avraham called the Holy Temple ‘mountain,’ as it says “On the mountain, the Lord will be seen.” (Genesis 22:14) Yitzchak called it field, as it says “…the fragrance of my son is like the fragrance of a field…” (Genesis 27:27) Yaakov called it ‘house’ even before it was built, as it says “This is none other than the house of God…” (Genesis 28:17) The Holy One said to him: by your life! You called it a house before it was built and I will call it by your name, as it says “Come, let us go up to the Lord's mount, to the house of the God of Jacob…” (Isaiah 2:3) So too Jeremiah said “So said the Lord: Behold I am returning the captivity of the tents of Jacob…” (Jeremiah 30:18) So too Asaf decorated his words and only recalled the God of Yaakov with a trumpet blast, as it says “…sound the shofar to the God of Jacob.” (Psalms 81:2)

To the chief musician, upon the Gittith, for the sons of Korah. This is what the scripture says in Isaiah 63:6, "I will tread down the peoples in my anger." What should we say? "I will make mention of the loving kindnesses of the Lord" (Isaiah 63:7). And anyone who sees the Gittith will be victorious. Isaiah saw it and said, "I will make mention of the loving kindnesses of the Lord." David saw it and said, "Upon Gittith" (Psalm 8:1). Asaph saw it and said, "Upon Gittith, a Psalm for Asaph" (Psalm 81:1). The sons of Korah saw it and said, "Upon the Gittith, for the sons of Korah."