Begin with a discussion, what is a fast day? Why do we fast? Is it a day of mourning, of tshuva? Why specifically do we fast n 17 Tamuz?
Five things befell our forefathers on the 17th of Tammuz: the Tablets were broken; the daily sacrifice ceased; the city (Jerusalem) was breached; Apostomus burned the Torah; and an idol was placed in the Temple.
(א) יֵשׁ שָׁם יָמִים שֶׁכָּל יִשְׂרָאֵל מִתְעַנִּים בָּהֶם מִפְּנֵי הַצָּרוֹת שֶׁאֵרְעוּ בָּהֶן כְּדֵי לְעוֹרֵר הַלְּבָבוֹת לִפְתֹּחַ דַּרְכֵי הַתְּשׁוּבָה וְיִהְיֶה זֶה זִכָּרוֹן לְמַעֲשֵׂינוּ הָרָעִים וּמַעֲשֵׂה אֲבוֹתֵינוּ שֶׁהָיָה כְּמַעֲשֵׂינוּ עַתָּה עַד שֶׁגָּרַם לָהֶם וְלָנוּ אוֹתָן הַצָּרוֹת. שֶׁבְּזִכְרוֹן דְּבָרִים אֵלּוּ נָשׁוּב לְהֵיטִיב שֶׁנֶּאֱמַר (ויקרא כו-מ) "וְהִתְוַדּוּ אֶת עֲוֹנָם וְאֶת עֲוֹן אֲבֹתָם" וְגוֹ':
(1) There are days on which all Israel fasts because of the misfortunes that occurred on them in order to arouse the hearts to open to the ways of repentance and that this be a reminder of our evil deeds and the deeds of our ancestors that are like our deeds that caused these misfortunes for them and for us. In remembering these things we return to goodness, as it is written, "And you shall confess your transgressions and the transgressions of your ancestors...(Leviticus 27:40)"
According to the Rambam are we being punished for our ancestor's sins? How does he explain this strange verse? What does he say about the purpose of fasting?
(א) מִצְוַת עֲשֵׂה מִן הַתּוֹרָה לִזְעֹק וּלְהָרִיעַ בַּחֲצוֹצְרוֹת עַל כָּל צָרָה שֶׁתָּבוֹא עַל הַצִּבּוּר. שֶׁנֶּאֱמַר (במדבר י-ט) "עַל הַצַּר הַצֹּרֵר אֶתְכֶם וַהֲרֵעֹתֶם בַּחֲצֹצְרוֹת". כְּלוֹמַר כָּל דָּבָר שֶׁיָּצֵר לָכֶם כְּגוֹן בַּצֹּרֶת וְדֶבֶר וְאַרְבֶּה וְכַיּוֹצֵא בָּהֶן זַעֲקוּ עֲלֵיהֶן וְהָרִיעוּ:
(1) It is a positive commandment from the Torah to cry out and to sound trumpets for all troubles that come upon the community, as it is said, "upon an enemy who attacks you and you sound trumpets (Numbers 10:9)." That is to say, every matter that troubles you like famine, plague, locusts, and so forth, cry out because of them and sound the trumpets.
(ב) וְדָבָר זֶה מִדַּרְכֵי הַתְּשׁוּבָה הוּא. שֶׁבִּזְמַן שֶׁתָּבוֹא צָרָה וְיִזְעֲקוּ עָלֶיהָ וְיָרִיעוּ יֵדְעוּ הַכּל שֶׁבִּגְלַל מַעֲשֵׂיהֶם הָרָעִים הוּרַע לָהֶן כַּכָּתוּב (ירמיה ה-כה) "עֲוֹנוֹתֵיכֶם הִטּוּ" וְגוֹ'. וְזֶה הוּא שֶׁיִּגְרֹם לָהֶם לְהָסִיר הַצָּרָה מֵעֲלֵיהֶם:
And this is one of the ways of repentance: that when trouble comes, they cry out because of it and sound the trumpets, and everyone knows that it is because of their evil deeds that things are bad for them… and it is this which causes them to remove the trouble from upon them.
Clearly the purpose of a fast is to change one's current ways