מדרש אויף ירמיהו 31:13

Then sang Moses. At times the word then refers to events that happened in the past, and at other times it refers to events that are to transpire in the future. The word then refers to past events in the following verses: Then began man to call upon the name of the Lord (Gen. 4:26), Then she said: “A bridegroom of blood” (Exod. 4:26), Then sang Israel this song (Num. 21:17), Then Solomon spoke (I Kings 18:12). All of these refer to past events. Instances in which the word then alludes to future events are: Then thou shalt see (Isa. 60:5), Then shall thy light break forth (ibid. 58:8), Then shall the lame man leap as a hart (ibid. 35:6), Then the eyes of the blind shall be opened (ibid., v. 5), Then shall the virgin rejoice (Jer. 31:13), Then was our mouth filled with laughter, and our tongue with singing (Ps. 126:2), and Then said they among the nations (ibid.). All these allude to future events. From this we learn that the concept of the resurrection of the dead is derived from the Torah.

The Holy One, blessed be He, said: In this world you grieved for this righteous man for seven days, but in the world-to-come I will turn their mourning into joy, and will comfort them, and make them rejoice from their sorrow (Jer. 31:13). And I will comfort Zion and its vast places, as it is said: For the Lord hath comforted Zion; He hath comforted all her waste places and hath made her wilderness like Eden, and her desert like the garden of the Lord; joy and gladness shall be found therein, thanksgiving and the voice of melody (Isa. 51:3).

{Another interpretation (of Gen. 49:28): AND THIS IS WHAT THEIR FATHER SPOKE TO THEM.} [(Gen. 50:12:) SO HIS CHILDREN DID FOR HIM JUST AS HE HAD COMMANDED THEM.] What did they do? They embalmed him and brought him up < to the land of Israel >.⁸²Tanh., Gen. 12:17; Gen. R. 97, New Version, on 49:29 (= pp. 1228-1229 in the Theodor-Albeck edition); Sot. 13a. What is written (in Gen. 50:10)? WHEN THEY CAME TO THE THORNBUSH THRESHING FLOOR.⁸³THE THORNBUSH THRESHING FLOOR is usually regarded as a proper name, i.e., as GOREN HA’ATAD. Is there a threshing floor for thorns, as it says: TO THE THORNBUSH THRESHING FLOOR? It is simply that these were Canaanites who had taken their crowns and surrounded Jacob's coffin, as a threshing floor when one surrounds it with thorns. R. Eliezer says: When they saw the coffin of our father Jacob, they undid the girdles on their waists and < so > showed honor to our father Jacob.⁸⁴Gen. R. 100(101):6; ySot. 1:10(17b). R. Samuel bar Nahman said: They undid their finger joints; and our masters say: They pointed the finger at Jacob's coffin and said (in Gen. 50:11): THIS IS A SOLEMN MOURNING FOR THE EGYPTIANS. For this reason the Holy One gave them a threshing floor.⁸⁵A sign of fruitfulness in what had been unfruitful, a mere thornbush threshing floor. (Gen. 50:10:) AND HE CARRIED OUT SEVEN DAYS OF MOURNING FOR HIS FATHER. From here we learn that the time for mourning is seven days. They also give evidence for it from here (in Lev. 8:33): AND YOU SHALL NOT GO OUT FROM THE DOOR OF THE TENT OF MEETING FOR SEVEN DAYS. And why do they carry out seven days of mourning rites? < It > corresponds to the seven days of feasting (on a joyous occasion). The Holy One said to them: In this world you have exhibited sorrow in the company of this righteous man and observed a seven-day mourning for him. In the world to come I am turning this mourning into feasting, as stated (in Jer. 31:13): FOR I WILL TURN THEIR MOURNING INTO JOY, COMFORT THEM, AND CHEER THEM IN THEIR GRIEF. Then, just as I comfort them, so I will give comfort to Zion, as stated (in Is. 51:3): TRULY THE LORD HAS COMFORTED ZION; HE HAS COMFORTED ALL ITS RUINS. HE HAS ALSO MADE ITS DESERT LIKE EDEN AND ITS DRY PLAIN LIKE THE GARDEN OF THE LORD. JOY AND REJOICING SHALL BE FOUND IN IT, THANKSGIVING AND THE SOUND OF MUSIC.