We are grateful to Gabriel Wasserman for contributing a transcription of this piyyut, and to Nir Karakauer for translating it. The piyyut is based on the Babylonian Talmud Taanit 30b-31a:

Why should the 15th of Av be a day of rejoicing? Said Rav Yehudah in the name of Shmuel: “On that day it was permitted to the members of the different tribes to intermarry.” Whence is this deduced? Because it is written [Numb. 36:6]: “This is the thing which YHVH has commanded concerning the daughters of Zelopheḥad,” etc., they claim that “this is the thing” implies that only for that generation was it decreed, but for later generations the decree does not apply.

Rav Yoseph in the name of Rav Naḥman said: On that day the members of the tribe of Benyamin were permitted to intermarry with the other tribes, as it is written [Judges 21:1]: “Now the men of Yisrael had sworn in Mitspah, saying: Not any one of us shall give his daughter unto Binyamin for wife.” Whence was it deduced that subsequently permission might be given to intermarry with the tribe of Benyamin? Because the quoted passage says “Any one of us,” and Rav said that their descendants were not included in the vow.

Rabba bar bar Ḥana said in the name of Rebbi Yoḥanan: On that day the last of those who were destined to die in the desert died, and the destiny was thus fulfilled; for the Master said that so long as the destiny was still unfulfilled, YHVH did not speak to Mosheh for his particular sake, as it is written [Deut. 2:16-17]: “So it came to pass, when all the men of war were spent by dying from the midst of the people, that YHVH spoke unto me, saying”; and “unto me” signifies that YHVH spoke unto Mosheh in particular.

Ula said: “On that day the guards appointed by Yeroboam to prevent the Israelites from coming to Jerusalem were abolished by Hoshea the son of Elah, and he said: ‘Let them go wherever they choose.'”

Rav Mattanah said: “On that day permission was given to bury the dead who were killed in battle at the city of Beitar.” And Rav Mattanah said again: “On that day, when it was permitted to bury those killed at Beitar, the assembly at Yamnia ordained the benediction reading: ‘Blessed are you, YHVH the good, that does good.’ What is meant thereby? By ‘good’ is meant that the bodies were not left to putrefy, and by ‘does good’ that burial was permitted.”

Rabba and Rav Yoseph both said: On that day they ceased to cut wood for the altar, as we have learned in a Baraita: Rebbi Eliezer the Great said: “From the fifteenth day of Av the heat of the sun was lessened and the timber was no longer dry, so they ceased to cut wood for the altar.” [Said Rav Menashya: “That day was called the day on which the axes were broken”], and from that day on, he who adds the night to his time for study may have years and days added to his life.

(adapted from the Rodkinson translation)

The refrain and last stanza also allude to Isaiah 40, read as the haftarah of the shabbat after Tishah b’Av, with its theme of consolation from the exile.