No Greaters Days For Israel Than Tu BeAv and Yom Kippur

So Tu BeAv is a great day for Israel, and for shidduchim. To cite Taanit 26b:

{Taanit 26b}

אמר רבן שמעון בן גמליאל לא היו ימים טובים לישראל כחמשה עשר באב וכיום הכפורים שבהם בנות ישראל יוצאות בכלי לבן שאולין שלא לבייש את מי שאין לו וכל הכלים טעונין טבילה ובנות ישראל יוצאות וחולות בכרמים ומה היו אומרות בחור שא עיניך וראה מה אתה בורר לך אל תתן עיניך בנוי תן עיניך במשפחה:

Rabban Shimon ben Gamliel said: There were not festivals for Israel comparable to the 15th of Av and Yom Kippur, that on these days the Jewish maidens would go out with borrowed white garments, so as not to embarrass she who has none. And all this clothing had to be previously immersed. And the Jewish maidens would go out and dance in the vineyards, and what did they say? "Young man, lift up your eyes and see what you choose for yourself. Do not cast your eyes on beauty but rather cast your eyes on family."

After the war sparked by the incident with the concubine of Gibeah, the remaining men of Banjamin could not marry, because of an oath everyone else took not to give a daughter to marry them. After the war, they did not want an entire tribe of Binyamin to die out, so they sought creative halachic solutions. First, they gave them the daughters of Yavesh Gilead, who had not partaken in the oath. Then, in the second half of Shoftim 21:

טז וַיֹּאמְרוּ זִקְנֵי הָעֵדָה, מַה-נַּעֲשֶׂה לַנּוֹתָרִים לְנָשִׁים: כִּי-נִשְׁמְדָה מִבִּנְיָמִן, אִשָּׁה.	16 Then the elders of the congregation said: 'How shall we do for wives for them that remain, seeing the women are destroyed out of Benjamin?'
יז וַיֹּאמְרוּ, יְרֻשַּׁת פְּלֵיטָה לְבִנְיָמִן; וְלֹא-יִמָּחֶה שֵׁבֶט, מִיִּשְׂרָאֵל.	17 And they said: 'They that are escaped must be as an inheritance for Benjamin, that a tribe be not blotted out from Israel.
יח וַאֲנַחְנוּ, לֹא נוּכַל לָתֵת-לָהֶם נָשִׁים--מִבְּנוֹתֵינוּ: כִּי-נִשְׁבְּעוּ בְנֵי-יִשְׂרָאֵל, לֵאמֹר, אָרוּר, נֹתֵן אִשָּׁה לְבִנְיָמִן. {ס}	18 Howbeit we may not give them wives of our daughters.' For the children of Israel had sworn, saying: 'Cursed be he that giveth a wife to Benjamin.' {S}
יט וַיֹּאמְרוּ הִנֵּה חַג-יְהוָה בְּשִׁלוֹ מִיָּמִים יָמִימָה, אֲשֶׁר מִצְּפוֹנָה לְבֵית-אֵל מִזְרְחָה הַשֶּׁמֶשׁ, לִמְסִלָּה, הָעֹלָה מִבֵּית-אֵל שְׁכֶמָה--וּמִנֶּגֶב, לִלְבוֹנָה.	19 And they said: 'Behold, there is the feast of the LORD from year to year in Shiloh, which is on the north of Beth-el, on the east side of the highway that goeth up from Beth-el to Shechem, and on the south of Lebonah.'
כ ויצו (וַיְצַוּוּ), אֶת-בְּנֵי בִנְיָמִן לֵאמֹר: לְכוּ, וַאֲרַבְתֶּם בַּכְּרָמִים.	20 And they commanded the children of Benjamin, saying: 'Go and lie in wait in the vineyards;
כא וּרְאִיתֶם, וְהִנֵּה אִם-יֵצְאוּ בְנוֹת-שִׁילוֹ לָחוּל בַּמְּחֹלוֹת, וִיצָאתֶם מִן-הַכְּרָמִים, וַחֲטַפְתֶּם לָכֶם אִישׁ אִשְׁתּוֹ מִבְּנוֹת שִׁילוֹ; וַהֲלַכְתֶּם, אֶרֶץ בִּנְיָמִן.	21 and see, and, behold, if the daughters of Shiloh come out to dance in the dances, then come ye out of the vineyards, and catch you every man his wife of the daughters of Shiloh, and go to the land of Benjamin.
כב וְהָיָה כִּי-יָבֹאוּ אֲבוֹתָם אוֹ אֲחֵיהֶם לרוב (לָרִיב) אֵלֵינוּ, וְאָמַרְנוּ אֲלֵיהֶם חָנּוּנוּ אוֹתָם--כִּי לֹא לָקַחְנוּ אִישׁ אִשְׁתּוֹ, בַּמִּלְחָמָה: כִּי לֹא אַתֶּם נְתַתֶּם לָהֶם, כָּעֵת תֶּאְשָׁמוּ. {ס}	22 And it shall be, when their fathers or their brethren come to strive with us, that we will say unto them: Grant them graciously unto us; because we took not for each man of them his wife in battle; neither did ye give them unto them, that ye should now be guilty.' {S}
כג וַיַּעֲשׂוּ-כֵן, בְּנֵי בִנְיָמִן, וַיִּשְׂאוּ נָשִׁים לְמִסְפָּרָם, מִן-הַמְּחֹלְלוֹת אֲשֶׁר גָּזָלוּ; וַיֵּלְכוּ, וַיָּשׁוּבוּ אֶל-נַחֲלָתָם, וַיִּבְנוּ אֶת-הֶעָרִים, וַיֵּשְׁבוּ בָּהֶם.	23 And the children of Benjamin did so, and took them wives, according to their number, of them that danced, whom they carried off; and they went and returned unto their inheritance, and built the cities, and dwelt in them.
כד וַיִּתְהַלְּכוּ מִשָּׁם בְּנֵי-יִשְׂרָאֵל בָּעֵת הַהִיא, אִישׁ לְשִׁבְטוֹ וּלְמִשְׁפַּחְתּוֹ; וַיֵּצְאוּ מִשָּׁם, אִישׁ לְנַחֲלָתוֹ. {פ}	24 And the children of Israel departed thence at that time, every man to his tribe and to his family, and they went out from thence every man to his inheritance. {P}
כה בַּיָּמִים הָהֵם, אֵין מֶלֶךְ בְּיִשְׂרָאֵל: אִישׁ הַיָּשָׁר בְּעֵינָיו, יַעֲשֶׂה. {ש}	25 In those days there was no king in Israel; every man did that which was right in his own eyes. {P}

It is quite possible that this last verse, which is also the last verse of the sefer, is passing negative judgment on these particular actions.

This incident has rough parallels with the account of the Rape of the Sabine women by the Romans -- the "rape" in that case connoting "seizing" (rapere, to grab) in order to wed. To cite Wikipedia:

It refers to an event supposed to have occurred in the early history of Rome, shortly after its foundation by Romulus and a group of mostly male followers. Seeking wives in order to found families, the Romans negotiated with the Sabines, who populated the area. The Sabines refused to allow their women to marry the Romans, fearing the emergence of a rival culture. Faced with the extinction of their community, the Romans planned to abduct Sabine women. Romulus invited Sabine families to a festival of Neptune Equester. At the meeting he gave a signal, at which the Romans grabbed the Sabine women and fought off the Sabine men. The indignant abductees were implored by Romulus to accept Roman husbands.

Here too, a seizing at the festival in order to allow an otherwise forbidden marriage.

What was the nature of this Jewish festival, and dancing? Was it really originally done for the purpose of marriage? It would seem not. Rather, festivals were a time of joy, and this was a way of expressing such joy. And it was by Shiloh (think Mishkan Shiloh), and it was a feast to Hashem:

יט וַיֹּאמְרוּ הִנֵּה חַג-ה בְּשִׁלוֹ מִיָּמִים יָמִימָה, אֲשֶׁר מִצְּפוֹנָה לְבֵית-אֵל מִזְרְחָה הַשֶּׁמֶשׁ, לִמְסִלָּה, הָעֹלָה מִבֵּית-אֵל שְׁכֶמָה--וּמִנֶּגֶב, לִלְבוֹנָה.

19 And they said: 'Behold, there is the feast of the LORD from year to year in Shiloh, which is on the north of Beth-el, on the east side of the highway that goeth up from Beth-el to Shechem, and on the south of Lebonah.'

It was presumably the unmarried virgins who went out to dance, which is why it was easy to seize the ones who went out to dance and take them as wives. Thus, perhaps there was some cultic aspect to these dances -- praising God via joy, celebration, and dance, and not just via sacrifice.

We have Tehillim 150:

א הַלְלוּ-יָהּ: הַלְלוּ-אֵל בְּקָדְשׁוֹ; הַלְלוּהוּ, בִּרְקִיעַ עֻזּוֹ.	1 Hallelujah. {N} Praise God in His sanctuary; praise Him in the firmament of His power.
ב הַלְלוּהוּ בִגְבוּרֹתָיו; הַלְלוּהוּ, כְּרֹב גֻּדְלוֹ.	2 Praise Him for His mighty acts; praise Him according to His abundant greatness.
ג הַלְלוּהוּ, בְּתֵקַע שׁוֹפָר; הַלְלוּהוּ, בְּנֵבֶל וְכִנּוֹר.	3 Praise Him with the blast of the horn; praise Him with the psaltery and harp.
ד הַלְלוּהוּ, בְּתֹף וּמָחוֹל; הַלְלוּהוּ, בְּמִנִּים וְעֻגָב.	4 Praise Him with the timbrel and dance; praise Him with stringed instruments and the pipe.
ה הַלְלוּהוּ בְצִלְצְלֵי-שָׁמַע; הַלְלוּהוּ, בְּצִלְצְלֵי תְרוּעָה.	5 Praise Him with the loud-sounding cymbals; praise Him with the clanging cymbals.
ו כֹּל הַנְּשָׁמָה, תְּהַלֵּל יָהּ: הַלְלוּ-יָהּ.	6 Let every thing that hath breath praise the LORD. {S} Hallelujah. {P}

Perhaps it developed it connection to matchmaking secondarily, as an outgrowth of the Biblical story.

Presumably, when the women went out to dance at Tu BeAv and Yom Kippur, the men did not just seize them there and that was all. Rather, presumably there was betrothal and a year later nuptials. And also most likely dating and arrangement of the match before that - shidduchin. But this was essentially a big singles event, in which men saw the available women and then sought their hand in marriage later.